fix(datasource/maven): Maven Central has no index.html

[aikebah]

Fixes the current misbehaviour of the maven datasource requesting index.html at the package-level, whereas the intended html page is served for the folder. Maven Central always responds HTTP 404 when requested for index.html

Changes

Change the maven datasource from requesting the non-existent <package-root-folder>/index.html to the proper <package-root-folder> to obtain the intended HTML-directory-listing of versions and their timestamps.

Updates the related testcases accordingly.

Context

As indicated in my remark on discussion 25461 the first remark made in the opening post, that index.html is non-existent was never addressed and still existent in the codebase of renovate resulting in every attempt for getting releases from Maven Central running into HTTP 404 errors.

Following up on the response given this is the PR that would resolve that issue.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 39.9.1 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 39.9.1

Your semantic-release bot 📦🚀