Update scaladocs mentioning deprecated API

Ensure nothing mentions the deprecated API removed in previous commits
for DAFFODIL-2743.  Also fix some IDEA nits and bump actions/setup-java
from 3.7.0 to 3.8.0 because GitHub has suddenly removed 3.7.0.

main.yml: Bump actions/setup-java from 3.7.0 to 3.8.0 because GitHub has
suddenly removed 3.7.0, meaning CI now breaks until we bump to 3.8.0.

DataProcessor.scala: Rewrite scaladoc mentioning compilerExternalVars
to say this constructor is for reconstructing a DataProcessor from a
SerializedDataProcessor. Remove another scaladoc since it's no longer
relevant.

SchemaSet.scala: Simplify scaladoc to stop mentioning deprecated API.
Fix some IDEA nits in the same code block with IDEA quick fixes.

DAFFODIL-2743

.github/workflows/main.yml

@@ -123,7 +123,7 @@ jobs:
           fetch-depth: 0
 
       - name: Setup Java
-        uses: actions/setup-java@v3.7.0
+        uses: actions/setup-java@v3.8.0
         with:
           distribution: ${{ matrix.distribution }}
           java-version: ${{ matrix.java_version }}

daffodil-core/src/main/scala/org/apache/daffodil/dsom/SchemaSet.scala

@@ -342,13 +342,9 @@ final class SchemaSet private (
   }
 
   /**
-   * The root element can be specified by a deprecated API call on the compiler
-   * object or the ProcessorFactory class, but the call on the ProcessorFactory class
-   * just overrides anything coming from the compiler object.
+   * You can define the root by passing the root specification to the Compiler.compileX method.
    *
-   * The right way is to pass the root specification to the Compiler.compileX method.
-   *
-   * Or, you can leave it unspecified, and this method will determine from the
+   * Or, you can leave the root unspecified, and this method will determine it from the
    * first element declaration of the first schema file.
    */
   lazy val root: Root = {
@@ -360,12 +356,12 @@ final class SchemaSet private (
           // if the root element and rootNamespace aren't provided at all, then
           // the first element of the first schema document is the root
           val sDocs = this.allSchemaDocuments
-          assuming(sDocs.length > 0)
-          val firstSchemaDocument = sDocs(0)
+          assuming(sDocs.nonEmpty)
+          val firstSchemaDocument = sDocs.head
           val gdecl = firstSchemaDocument.globalElementDecls
           val firstElement = {
-            schemaDefinitionUnless(gdecl.length >= 1, "No global elements in: " + firstSchemaDocument.uriString)
-            gdecl(0)
+            schemaDefinitionUnless(gdecl.nonEmpty, "No global elements in: " + firstSchemaDocument.uriString)
+            gdecl.head
           }
           firstElement
         }

daffodil-runtime1/src/main/scala/org/apache/daffodil/processors/DataProcessor.scala

@@ -154,27 +154,23 @@ class DataProcessor private (
    * That means when we save for reloading, we must explicitly clobber validationMode and externalVars to
    * initialized values.
    *
-   * @throws java.io.ObjectStreamException
+   * @throws java.io.ObjectStreamException Must be part of writeReplace's API
    * @return the serializable object
    */
   @throws(classOf[java.io.ObjectStreamException])
   private def writeReplace() : Object =
     new SerializableDataProcessor(ssrd, tunables, externalVars, validationMode)
 
   /**
-   * The compilerExternalVars argument supports the deprecated feature to assign external var bindings
-   * on the compiler object.
-   *
-   * These are just incorporated into the initial variable map of the data processor.
+   * This constructor reconstructs a DataProcessor from a SerializedDataProcessor.
    */
-
   def this(
     ssrd: SchemaSetRuntimeData,
     tunables:DaffodilTunables,
-    compilerExternalVars: Queue[Binding] = Queue.empty,
+    externalVars: Queue[Binding] = Queue.empty,
     validationMode: ValidationMode.Type = ValidationMode.Off) =
-    this(ssrd, tunables, ExternalVariablesLoader.loadVariables(compilerExternalVars, ssrd, ssrd.originalVariables),
-      false, None, validationMode, compilerExternalVars)
+    this(ssrd, tunables, ExternalVariablesLoader.loadVariables(externalVars, ssrd, ssrd.originalVariables),
+      false, None, validationMode, externalVars)
 
   def copy(
     ssrd: SchemaSetRuntimeData = ssrd,
@@ -271,13 +267,6 @@ class DataProcessor private (
     copy(externalVars = newBindings)
   }
 
-  /**
-   * Note that tunables is not used. So this method is equivalent to
-   * the other similar method that doesn't take that parameter.
-   *
-   * @param extVars File containing configuration with external variable bindings in it.
-   * @param tunable This is ignored.
-   */
   def withExternalVariables(extVars: Seq[Binding]): DataProcessor = {
     val newBindings = loadExternalVariables(extVars)
     copy(externalVars = newBindings)