MINOR: Small cleanups in connect/mirror

[mimaison]

• Make a few fields final
• Remove unnecessary throws

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[divijvaidya]

Hey @mimaison, one of the flaky tests failing for this PR is testListenerConnectionRateLimitWhenActualRateAboveLimit() – kafka.network.ConnectionQuotasTest which I have fixed as part of #12045

If you get a chance, please review my PR and that will reduce some degree of flakiness from the existing test suite.

Thanks in advance!

[showuon]

LGTM! Thanks for the PR.

[showuon]

Failed tests are unrelated:

Build / JDK 17 and Scala 2.13 / org.apache.kafka.streams.integration.NamedTopologyIntegrationTest.shouldAddToEmptyInitialTopologyRemoveResetOffsetsThenAddSameNamedTopology
Build / JDK 8 and Scala 2.12 / kafka.server.KRaftClusterTest.testCreateClusterAndPerformReassignment()
Build / JDK 11 and Scala 2.13 / kafka.network.ConnectionQuotasTest.testListenerConnectionRateLimitWhenActualRateAboveLimit()