Rename wp-cli commands

[MARQAS]

Description of the Change

This PR is used to rename the WP-CLI commands as requested in #2981

Closes #2981

How to test the Change

• Run the old WP-CLI commands, they will give a deprecation warning along with the expected output
• Run the renamed commands, they will give the expected output

Changelog Entry

Added - get-cluster-indices, get-indices, sync, clear-sync, get-ongoing-sync-status, get-last-cli-sync, stop-sync WP-CLI commands
Deprecated - get-cluster-indexes, get-indexes, index, clear-index, get-indexing-status, get-last-cli-index, stop-indexing WP-CLI commands

Credits

Props @MARQAS

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

@MARQAS I've been thinking about this one and before we merge it there is a couple of changes we will need to make here:

1. To avoid breaking someone else's code that can be extending our Command class, instead of a whole new class, let's make DeprecatedCommand a trait and include it in the regular Command class. This way we keep methods in a separate file but they will still be available to others.
2. The _deprecated_function function does not output a warning to the user. Let's keep it there but also make a call to WP_CLI::warning and add a notice to the command doc. Examples:
   https://github.com/wp-cli/language-command/blob/8e25528afc90b709aaebba429752a86d12c500d1/src/Core_Language_Command.php#L334
   https://github.com/wp-cli/scaffold-command/blob/4ae0066a64db0b93b4f923554cb8ab2b921e16c9/src/Scaffold_Command.php#L207

[felipeelia]

After 93e47a3, this still needs to be done (all in DeprecatedCommand.php):

1. Remove all @alias from the commands
2. Add WP_CLI::warning() calls to each command
3. Update the documentation of each command adding preceding the main desc. with DEPRECATED and adding the alternative to the long description
4. Test it (manual testing should be enough for this, so we don't bloat our test suites with deprecated stuff)
5. Fix the documentation of clear_index to include $args and $assoc_args
6. Fix git conflicts