[9.0](backport #15976) Change server information endpoint / to only accept GET and HEAD requests

[mergify]

Motivation/summary

Breaking change to change server information endpoint / to only accept GET and HEAD requests, and return 405 Method Not Allowed otherwise. This will surface any agent misconfiguration, e.g. configuring otlphttp to send to / instead of /v1/traces.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated: apm-server: Remove server information endpoint POST / support docs-content#647

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

curl -XGET -v http://apm-server/
curl -XHEAD -v http://apm-server/
curl -XPOST -v http://apm-server/

Confirm that GET and HEAD requests return 200, POST requests return 405.

Related issues

Fixes #15965

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This is an automatic backport of pull request #15976 done by [Mergify](https://mergify.com).