Deprecate withers in 2.x in favor of setters

[jhl221123]

This pull request addresses issue #3750 by deprecating all withers in builder classes and introducing setters as replacements. All internal usages and related tests have been updated to use the setters.

Important

Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise.

Checklist

Before we can review and merge your changes, please go through the checklist below. If you're still working on some items, feel free to submit your pull request as a draft—our CI will help guide you through the remaining steps.

✅ Required checks

• License: I confirm that my changes are submitted under the Apache License, Version 2.0.

• Commit signatures: All commits are signed and verifiable. (See GitHub Docs on Commit Signature Verification).

• Code formatting: The code is formatted according to the project’s style guide.

  How to check and fix formatting

  • To check formatting: ./mvnw spotless:check
  • To fix formatting: ./mvnw spotless:apply

  See the build instructions for details.

• Build & Test: I verified that the project builds and all unit tests pass.

  How to build the project

  Run: ./mvnw verify

  See the build instructions for details.

🧪 Tests (select one)

• I have added or updated tests to cover my changes.
• No additional tests are needed for this change.

📝 Changelog (select one)

• I added a changelog entry in src/changelog/.2.x.x. (See Changelog Entry File Guide).
• This is a trivial change and does not require a changelog entry.