docs: update apmschema url to use apm-data

[kruskall]

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

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

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• May the instrumentation collect sensitive information, such as secrets or PII (ex. in headers)?
  • Yes
    • Add a section to the spec how agents should apply sanitization (such as sanitize_field_names)
  • No
    • Why?
  • n/a
• Create PR as draft
• Approval by at least one other agent
• Mark as Ready for Review (automatically requests reviews from all agents and PM via CODEOWNERS)
  • Remove PM from reviewers if impact on product is negligible
  • Remove agents from reviewers if the change is not relevant for them
• Approved by at least 2 agents + PM (if relevant)
• Merge after 7 days passed without objections
  To auto-merge the PR, add /schedule YYYY-MM-DD to the PR description.
• Create implementation issues through the meta issue template (this will automate issue creation for individual agents)
• If this spec adds a new dynamic config option, add it to central config.

Related to elastic/apm-server#17707