[ILM] Error when saving built-in policies with _meta field

[yuliacech]

Kibana version:
reproducible on v7.14, 7.15, 7.x and master branches
Elasticsearch version:

Server OS version:

Browser version:

Browser OS version:

Original install method (e.g. download page, yum, from source, etc.):

Describe the bug:
Built-in ILM policies can't be updated via UI because a new _meta property added in #73629 is being blocked by Kibana server validation.
Steps to reproduce:

1. Navigate to Index Lifecycle Policies app, click to edit "metrics" ILM policy or any other that has a _meta property
2. Click "save policy" button
3. Instead of the policy being saved, there is an error notification

Expected behavior:

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

{"statusCode":400,"error":"Bad Request","message":"[request body._meta]: definition for this key is missing"}

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-stack-management (Team:Stack Management)