Undeprecate metrics, logs and synthetics ILM policies

[MichelLosier]

As per the discussion in the linked issue, this PR undeprecates the legacy ILM policies metrics, logs, and synthetics. Package installs have not used the current @lifecycle named versions of these, and since users can and have modified the legacy ILM policies, migrating users to the @lifecycle would introduce too much risk.

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[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[elasticsearchmachine]

Hi @MichelLosier, I've created a changelog YAML for you.

[felixbarny]

This un-deprecates a lot more than just the ILM policies we talked about.

[MichelLosier]

You're right, my mistake this PR does affect other deprecated templates -- I misread their relationship here with the ILM policies.

Aside from that, I original thought the scope of this could be any ILM policy type declared in a package that follows the @lifecycle convention, and so all legacy policies replaced by @lifecycle ones would be implicated, but reviewing package-spec we do enumerate specifically what ILM policies can be used.

• https://github.com/elastic/package-spec/blob/main/spec/integration/data_stream/manifest.spec.yml#L534-L538
  metrics, logs, synthetics, traces, and profiling

Reviewing current integrations, I just see metrics, logs, and synthetics in use. So I can rescope this to just metrics, logs and synthetics ILM policies. On the kibana side, we can append @lifecycle to all other ILM policies (traces, profiling et c.).

Next steps:

I'm going to put this back in draft, and just move the deprecated metrics, logs, and synthetics into StackTemplateRegistry and revert deprecated back to true for LegacyStackTemplateRegistry.

[dakrone]

Before we merge this, I'd like to discuss with Felix about if there is a better way we can support this without having two copies of the policies existing in perpetuity.

[MichelLosier]

After further discussion, we don't want to put ourselves in a position where we have both legacy and current ILM policies in place which may also confuse the user. We'll look to provide a migration experience instead for users and possibly look at removal for the next major ver.