Update contribution guidelines for version targeting

[akarnokd]

Clarify contribution guidelines regarding version targeting.

Thank you for contributing to RxJava. Before you create it, please consider these:

• Describe what your contribution is about in English. No need for a verbose explanation disproportional to the impact of the change.
• Make sure you followed the code style of the repository and you have run the validators unit tests locally that enforce most of it.
• Make sure your change is accompanied by the relevant unit tests. If you are unsure how to test, indicate it and we'll help you.
• If you are responding to an issue, please include its # number in the commit text: Resolves #xxxx or Related #xxxx.
• You have to disclose if you used AI to generate the changes. We'd also like you disclosed if you used AI to come up with the fix idea but coded it yourself.
• Please avoid unsolicited pedantic or objectively low effort PRs or anything that would constitute profile propping.
• Do not post fire-and-forget PRs and we will not argue with your AI how to fix the mistakes.
• We are a modern repo, failing to repeatedly heed expected feedbacks will result in a shame + ban.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.653 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

CONTRIBUTING.md: 4.653

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.64%. Comparing base (556565f) to head (09f5064).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8316      +/-   ##
============================================
+ Coverage     98.62%   98.64%   +0.01%     
  Complexity     7157     7157              
============================================
  Files           824      824              
  Lines         50801    50801              
  Branches       6914     6914              
============================================
+ Hits          50104    50113       +9     
  Misses          474      474              
+ Partials        223      214       -9     

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