Relates to #22024
On top of documentation, the PR adds deprecation loggers and deals with the resulting warning headers.
The yaml test is set exclude versions up to 6.0. This is need to make sure bwc tests pass until this is backported to 5.2.0 . Once that's done, I will change the yaml test version limits
0219a211d3539781ad4bc59bbb68ab9dd5633090 added support for templates
to have multiple patterns and renamed `template` to `index_patterns`.
This adds the breaking changes docs for that.
