Update 4.x Getting Started documentation

[evgenyponomarev]

Summary

• replace hard-coded dependency versions with a reusable x.y.z placeholder and remove the unrelated RxScala dependency
• make the download-only POM use neutral project coordinates instead of stale RxJava 3 metadata
• update the required Java version from 6 to 26
• point snapshot consumers to the current Central repository and use HTTPS for snapshot Javadocs

This addresses the Getting-Started.md item in #6132 without closing the tracking issue.

Validation

• git diff --check
• verified 4.0.0-SNAPSHOT/maven-metadata.xml at the documented Central repository returns HTTP 200
• ./gradlew test --tests "io.reactivex.rxjava4.validators.*" --stacktrace --no-daemon
• ./gradlew javadoc --stacktrace
• ran ./gradlew build --stacktrace twice; each 14,214-test run encountered a different unrelated concurrency failure (CompletableIsolatedTest.repeatNormal, then ParallelSchedulerTests.unSubscribeForScheduler), and both failing methods passed when rerun in isolation

AI disclosure

This contribution was made with OpenAI Codex assistance. Codex compared the 4.x Getting Started page with the current README, Gradle configuration, release list, and snapshot workflow; applied the documentation-only update; checked the published URLs; and ran the validation listed above. The reasoning was to remove stale RxJava 3/project-external examples and align the page with the repository’s current Java 26 and Maven Central configuration.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 5.161 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

docs/Getting-Started.md: 5.161

✅ 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 98.61%. Comparing base (4e481d5) to head (b30f66f).
⚠️ Report is 1 commits behind head on 4.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8290      +/-   ##
============================================
+ Coverage     98.60%   98.61%   +0.01%     
- Complexity     7150     7151       +1     
============================================
  Files           824      824              
  Lines         50791    50791              
  Branches       6911     6911              
============================================
+ Hits          50084    50090       +6     
+ Misses          482      478       -4     
+ Partials        225      223       -2     

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