Update JDKs using which the project is built

[Stephan202]

Suggested commit message:

Update JDKs using which the project is built (#545)

- Perform a regular build using JDK 20 instead of JDK 19.
- Perform an experimental build using JDK 21-ea instead of JDK 20-ea.

[github-actions]

Looks good. No mutations were possible for these changes.
Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[Stephan202]

The JDK 21-ea build fails due to an incompatibility with Error Prone 2.18.0. Requires a closer look.

[Stephan202]

When locally building against the latest revision of Error Prone, the JDK 21-ea errors seen here go away. But then the build fails due to an incompatibility in NullAway:

error-prone version: HEAD-SNAPSHOT
BugPattern: (see stack trace)
Stack Trace:
java.lang.NoClassDefFoundError: com/sun/tools/javac/code/TypeMetadata$Entry
    at com.uber.nullaway.NullAway.matchParameterizedType(NullAway.java:682)
    at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:449)
    at com.google.errorprone.scanner.ErrorProneScanner.visitParameterizedType(ErrorProneScanner.java:783)
    at com.google.errorprone.scanner.ErrorProneScanner.visitParameterizedType(ErrorProneScanner.java:150)
    at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCTypeApply.accept(JCTree.java:2825)
    at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:92)

[Stephan202]

I forgot that I had opened this PR; in the mean time I filed #604. Will close this one,