RFR: 8288: Clean up warnings

[Marcus Hirt]

Cleaning up some of the warnings in core in preparation for hack-commit-push (https://switzerland2024.hack-commit-pu.sh/). The ones related to the security manager being deprecated will be taken care of in JMC 10 (https://bugs.openjdk.org/browse/JMC-8289).

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Commit messages:
 - 8288: Cleaning up warnings

Changes: https://git.openjdk.org/jmc/pull/604/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=604&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8288
  Stats: 81 lines in 29 files changed: 1 ins; 27 del; 53 mod
  Patch: https://git.openjdk.org/jmc/pull/604.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/604/head:pull/604

PR: https://git.openjdk.org/jmc/pull/604