Remove deprecated config options

[rubvs]

Motivation/summary

• OS Deprecation: Based on https://www.elastic.co/support/matrix support for MacOS 11 & 12 are deprecated in v9.
• Config Deprecation: Remove deprecated warnings that have been deprecated for some time now. For detail on the removal of these config options see:
  • Remove index management, always use data streams #6606
  • Update indexing-related configuration docs for standalone #6662

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

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

For the config deprecations:

apm-server:
  host: "127.0.0.1:8200"
  data_streams.enabled: false
  ilm.enabled: "auto"
  register.ingest.pipeline: false

output.console:
  enabled: true

with current main we get the following warnings

> ./apm-server -e -v -c apm-server.yml

{
  "log.level": "warn",
  "@timestamp": "2025-05-29T15:31:20.985-0400",
  "log.logger": "beater.index-management",
  "log.origin": {
    "function": "github.com/elastic/apm-server/internal/idxmgmt.logWarnings",
    "file.name": "idxmgmt/supporter_factory.go",
    "file.line": 61
  },
  "message": "`apm-server.data_streams.enabled` specified, but was removed in 8.0 and will be ignored: data streams are always enabled",
  "service.name": "apm-server",
  "ecs.version": "1.6.0"
}
{
  "log.level": "warn",
  "@timestamp": "2025-05-29T15:31:20.985-0400",
  "log.logger": "beater.index-management",
  "log.origin": {
    "function": "github.com/elastic/apm-server/internal/idxmgmt.logWarnings",
    "file.name": "idxmgmt/supporter_factory.go",
    "file.line": 61
  },
  "message": "`apm-server.ilm` specified, but was removed in 8.0 and will be ignored: ILM policies are managed by Fleet",
  "service.name": "apm-server",
  "ecs.version": "1.6.0"
}
{
  "log.level": "warn",
  "@timestamp": "2025-05-29T15:31:20.985-0400",
  "log.logger": "beater.index-management",
  "log.origin": {
    "function": "github.com/elastic/apm-server/internal/idxmgmt.logWarnings",
    "file.name": "idxmgmt/supporter_factory.go",
    "file.line": 61
  },
  "message": "`apm-server.register.ingest.pipeline` specified, but was removed in 8.0 and will be ignored: ingest pipelines are managed by Fleet",
  "service.name": "apm-server",
  "ecs.version": "1.6.0"
}

but these warnings does exist when running on this PR's branch.

Related issues

Closes #16707

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[mergify]

This pull request does not have a backport label. Could you fix it @rubvs? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.