Remove warm phase from default ILM policy

[axw]

Motivation/summary

Remove warm phase from default ILM policy for standalone. Having a warm phase by default causes warm data nodes to spin up after 30 days when autoscaling is enabled; this is surprising to users, and was unintended.

This does not affect existing deployments unless they explicitly opt into overwriting the ILM policy during server initialisation/setup. Users who wish to have a warm phase and have enabled overwriting will need to customise their ILM policy in apm-server.yml.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

How to test these changes

1. Run apm-server in a clean cluster
2. Check that the the ILM policy created does not include a warm phase

Related issues

Closes #6170

[axw]

@bmorelli25 FYI there are a few minor docs updates here.

[apmmachine]

💔 Tests Failed

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Build stats

• Start Time: 2021-10-12T05:51:21.933+0000

• Duration: 51 min 49 sec

• Commit: a763727

Test stats 🧪

Test	Results
Failed	1
Passed	6184
Skipped	18
Total	6203

Test errors

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Build and Test / ARM build-test / TestModelIndexerLogRateLimit – modelindexer

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 Failed 

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     indexer_test.go:256: 
    indexer_test.go:257:  

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[mergify]

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

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[marclop]

Verified using 7.16.0 BC1:

GET _ilm/policy/apm-rollover-30-days
{
  "policy": {
    "phases": {
      "hot": {
        "min_age": "0ms",
        "actions": {
          "rollover": {
            "max_size": "50gb",
            "max_age": "30d"
          },
          "set_priority": {
            "priority": 100
          }
        }
      }
    }
  }
}