docs: remove unused warnOldDocs.js and projectSpecificFooter template

[pjfanning]

Motivation

docs/src/main/paradox/assets/js/warnOldDocs.js is the Akka-era "you are reading an old version of the docs" banner script. It is dead code:

• Its only reference is docs/src/main/paradox/_template/projectSpecificFooter.st, a footer hook from Akka's own Paradox theme. The pekko-theme-paradox page.st/footer.st templates never include projectSpecificFooter, so no generated page loads the script. The published site confirms this: https://pekko.apache.org/docs/pekko/current/index.html only loads the theme's scripts, though the unused .js file is still copied into the site as a static asset.
• Even if loaded, it fetches //pekko.io/versions.json, which does not exist (pekko.io does not resolve; pekko.apache.org/versions.json is a 404), so the banner logic is a no-op.
• Its URL parsing assumes Akka's <project>/<version>/ path layout and does not match /docs/pekko/<version>/.

None of the other Pekko repos (pekko-http, pekko-connectors, pekko-grpc, pekko-persistence-jdbc, pekko-management) carry this file.

Modification

Delete warnOldDocs.js and _template/projectSpecificFooter.st (both directories are now empty and removed).

Result

Unused docs assets are gone. Generated docs are unchanged apart from no longer shipping the unused script.

Tests

• Not run locally - docs only; relying on the CI docs build.

References

None - cleanup of unused Akka-era docs assets.

[raboof]

a bit sad (we're still publishing old versions of the docs, e.g. https://pekko.apache.org/docs/pekko/1.6.0/ , and it's nice to point people to the latest version), but better to clean it up (and potentially resurrect later) than to leave it broken