[all][build] Fix spotbug bugs and enforce low report level

[adamxchen]

Summary

Our Github Action enables the SpotBugs check but ignores the failure.

1. In order to report bugs and fail the CI, this PR removes that flag to ensure bugs can be reported.
2. The PR changes the report level from low to high, as low level reports too many bugs and cannot be fixed at a short term. Report level stays at low as I've fixed most of bugs.
3. All ~2380 bugs reported are also resolved in this PR.

Apologies for the huge PR, however given most of changes are quite trivial, mostly related to missing final/static keyword, dead codes or minor details, I think it's better to include them all in this PR so we don't have to bug people to review multiple PRs.

What to review

For the review, please pay attention on these items:

1. Whether dead codes are still been used somewhere. I've checked them are not being used in our code base or partners' code bases but could still miss something
2. Whether excluded filter doesn't make sense
3. Whether any excluded bugs draw your attention and require a fix

Testing

./gradlew --continue --no-daemon -Pspotallbugs clean check -x spotlessCheck -x test -x integrationTest -x jacocoTestCoverageVerification

WcTest jdk11 7460/7462/7464/7502
WcTest jdk8 1145/46/47

[lluwm]

Adam, thank you for fixing all spotbug warnings! Nice work and it looks good to me.

[majisourav99]

LGTM. Thanks for the effort Adam!