docs: update apmschema url to use apm-data

[kruskall]

What does this pull request do?

apm schema was moved to apm-data a long time ago and the apm-server files are just a mirror

Why is it important?

update the script to use apm-data files to we can stop mirroring them in apm-server

Checklist

• I have signed the Contributor License Agreement.
• My code follows the style guidelines of this project (See .rubocop.yml)
• I have rebased my changes on top of the latest main branch

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

• I have made corresponding changes to the documentation
• I have updated docs/release-notes/index.md
• I have updated docs/reference/supported-technologies.md
• Added an API method or config option? Document in which version this will be introduced

Related issues

• Related to move spec to apm-data and stop syncing files apm-server#17707