Bump Java to 21 in JVM builder images

[vlsi]

Currently, JVM builder images ship with Java 21 which has drawbacks:

1. Java 17 compiler is stale, so it might have more known bugs than Java 21
2. Newer projects might require Java 21 for the build, so having only Java 17 in the builder image prevents building those

An alternative option could be including both Java 17 and Java 21.

See:

• https://github.com/google/oss-fuzz/tree/master/infra/base-images/base-builder-jvm