These settings were deprecated in 7.13+ in #72835 and are now removed by this commit.
This commit also ensures that the settings are removed from index metadata when the metadata is
loaded. The reason for this is that if we allow the settings to remain (because they are not
technically "invalid"), then the index will not be able to be allocated, because the
FilterAllocationDecider will be looking for nodes with the _tier attribute.
* Deprecate single-tier allocation filtering settings
`(index|cluster).routing.allocation.(include|exclude|require)._tier` settings are now deprecated in
favor of using `index.routing.allocation.include._tier_preference`.
* Update deprecation message
This adds general overview documentation for data tiers,
the data tiers specific node roles, and their application in
ILM.
Co-authored-by: Lee Hinman <dakrone@users.noreply.github.com>
Co-authored-by: debadair <debadair@elastic.co>
