Improve default behaviour for the security migrations minMappingVersion

[jfreden]

There isn't a requirement that security migrations need to involve an index mapping change. The framework currently forces an implementer of a migration to specify a min mapping version, which leads to confusion since the change might be completely unrelated to mappings.

We could instead rely entirely on node features and skip the mapping version check when not needed by adding a sane default implementation.

Relates to: #114830 (comment)

[elasticsearchmachine]

Pinging @elastic/es-security (Team:Security)