Replace `action.auto_create_index` with a template setting

[clintongormley]

In #9359 we discussed the possibility of extending the action.auto_create_index setting to automatically recognise index names which match templates, and to allow those to be auto-created without having to maintain both the template and the setting.

In FixitFriday we came up with a better solution:

• Remove the action.auto_create_index setting
• Add an auto_create_index setting to index templates - if true, then an index can be auto-created
• Ship with a default match * index with auto_create_index: true - this can be overridden by the user
• If multiple templates match, the auto_create_index setting is merged just like all other template settings - the highest order wins

When upgrading, the user will see an exception if they set action.auto_create_index and will need to update their templates to reflect the same logic.

Possibly, we could add a deprecation period where the action.auto_create_index setting continues to work, but also accepts a _template value which switches to use the template-based logic instead.

[elasticmachine]

Pinging @elastic/es-core-infra