Rename "elasticsearch" Graylog server properties

## What?

Several server configuration properties (supported in the `server.conf`) file have the word `elasticsearch` in them. These should be renamed to have the general `indexer` name instead.

See config files:
- https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/configuration/ElasticsearchConfiguration.java
- https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/configuration/ElasticsearchClientConfiguration.java

## Why?

With the added support of Opensearch, using the name `elasticsearch` in the properties is no longer correct. Renaming them to the general `indexer` name is probably best.

This issue was created as a continuation of the discussion in https://github.com/Graylog2/graylog2-server/pull/13719#issuecomment-1282281972

Note that there were previously deprecated `elasticsearch` indexer properties, which are no longer deprecated with PR https://github.com/Graylog2/graylog2-server/pull/13018 since the plan is to use those as initializer values for the first boot going forward. Also see https://github.com/Graylog2/graylog-plugin-enterprise/issues/4091#issuecomment-1255131604.

## Naming
As mentioned above, I think we should globally rename `elasticsearch_...` properties to `indexer_...`, but in addition, I suggest that those properties used to initialize the new in-database index set defaults in https://github.com/Graylog2/graylog2-server/pull/13018 (see the properties referenced in https://github.com/Graylog2/graylog2-server/blob/55f5ca2991858fb7b94be0eba1ac85fafe97b5b2/graylog2-server/src/main/java/org/graylog2/migrations/V202211021200_CreateDefaultIndexDefaultsConfig.java) also include `init_default` in their name to clearly indicate that those are used for index default initialization (on first boot only).

## Backwards compatibility
@todvora Added backwards compatible naming support for the configuration library in https://github.com/Graylog2/JadConfig/pull/113. So, we can use the new `fallbackPropertyName` option to maintain support for the previous names.

## Documentation updates
Note that documentation updates will also be required, since this page lists some supported configuration values:
https://docs.graylog.org/docs/server-conf

**Note: This content is planned for Graylog `5.1.0`.**