Update quickstart guide to JUnit 5

[Chandu0501]

Closes #257.

Summary

• switch the Java quickstart page from JUnit 4 rule usage to JUnit 5 Jupiter extension usage
• update the sidebar and getting-started labels so they match the new quickstart content
• make the examples use @wiremocktest and WireMockRuntimeInfo for fixed and random port cases

Verification

• pnpm.cmd install
• pnpm.cmd build (Astro rendered dist/docs/quickstart/java-junit/index.html and related docs successfully; the command still exits non-zero on Windows because of the repo's pre-existing copy-static-homepage hook trying to copy to C:\C:...\dist\index.html)