Enable running on Java 17 (#541)

Before this path, mdoc keeps crashing with
```
[info] running (fork) mdoc.Main 
[error] java.lang.NoSuchFieldException: ucp
[error] 	at java.base/java.lang.Class.getDeclaredField(Class.java:2549)
[error] 	at mdoc.internal.CompatClassloader$.getURLs(CompatClassloader.scala:29)
```
As seen for example here
https://github.com/eikek/emil/pull/233/checks?check_run_id=3260441241#step:6:450
eikek/emil#233

.github/workflows/ci.yml

@@ -37,7 +37,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: [[email protected], [email protected]]
+        java: ['[email protected]', '[email protected]', '[email protected]']
         command:
           - "'++2.11.12 test'"
           # Test legacy Scala versions, where reporting API changed

.jvmopts

@@ -3,5 +3,4 @@
 -Xmx2G
 -XX:ReservedCodeCacheSize=1024m
 -XX:+TieredCompilation
--XX:+CMSClassUnloadingEnabled
 -Dfile.encoding=UTF-8

mdoc-sbt/src/sbt-test/sbt-mdoc/basic/project/build.properties

@@ -0,0 +1 @@
+sbt.version = 1.5.5

mdoc/src/main/scala/mdoc/internal/CompatClassloader.scala

@@ -26,7 +26,14 @@ object CompatClassloader {
         val unsafe = field.get(null).asInstanceOf[Unsafe]
 
         // jdk.internal.loader.ClassLoaders.AppClassLoader.ucp
-        val ucpField = classLoader.getClass().getDeclaredField("ucp")
+        val ucpField = {
+          if (System.getProperty("java.version").split("\\.")(0).toInt >= 16) {
+            // the `ucp` field is  not in `AppClassLoader` anymore, but in `BuiltinClassLoader`
+            classLoader.getClass().getSuperclass()
+          } else {
+            classLoader.getClass()
+          }
+        }.getDeclaredField("ucp")
         val ucpFieldOffset: Long = unsafe.objectFieldOffset(ucpField)
         val ucpObject = unsafe.getObject(classLoader, ucpFieldOffset)