docs: Broken anchor in github release notes

[carsonip]

Github apm-server release notes e.g. 9.0.3 has a link like https://www.elastic.co/docs/release-notes/apm#apm-9.0.3-release-notes, note the anchor #apm-9.0.3-release-notes.

Issue:
In our docs, the actual anchor is #elastic-apm-9.0.1-release-notes, which was what was agreed upon with release engineering, and our tooling is also outdated which would produce #9.0.1-release-notes instead.

Action item:

• Check with release engineering whether they should fix the github link
• Otherwise, fix the anchor in docs/release-notes/index.md
• In any case, fix tools/scripts/changelog.sh to align with the agreed anchor