Java 21 migration

[VincentCucchietti]

Migrate to Java 21

Mirrors the Java 21 migration already applied to orekit, using the same OpenRewrite
recipe sequence, one recipe per commit, followed by the manual follow-ups needed to
make the build target 21 cleanly.

Recipes:

• Migrated to Java 17
• Upgraded build to Java 21
• Removed extra semicolons
• Migrated to SequencedCollection (kept indexed .get(i) access wherever it sits
  next to other indexed access on the same list, for consistency)
• Converted if-else construct to switch
• Converted switch case assignments to switch expression
• Converted switch case returns to switch expression

Follow-ups:

• Switched compiler source/target to a single hipparchus.compiler.release property
• Raised minimum required Java version (enforcer) to 21
• Bumped CI to Java 21