Add support for Silverstripe 5

[chrispenny]

RFC: Extract Elastic-specific classes into a separate module #81

This PR includes the removal of the Elastic integration, and this is now going to be provided by Silverstripe Search Service - Elastic module.

Breaking changes

• The Elastic integration classes will all have a new namespace. Any references you have will need to be updated.
• There have been some PHP 8.1 linting/declaration updates. If you have extended any of our classes, there might be some signature updates that you need to make.
• EnterpriseSearchService::environmentizeIndex() is no longer static, and this is now part of the contract for IndexingInterface. If you were referencing or overriding this method, then you might need to update your implementation.
• (Other than the above) there were no changes to processing logic.
• No method constructors or method property orders were changed.

Silverstripe 5 support

There actually wasn't much that needed to change. The default property value in DataObjectFetcher was the most significant change.

[chrispenny]

@madmatt keen to get your thoughts on this. I still need to run through manual testing, but I think all major changes have now been made.

In particular, I really don't know anything about how Algolia configuration would work, and I think this module is (currently) quite opinionated in how it expects you to provide your index configuration.

CC: @GuySartorelli

[GuySartorelli]

I haven't done a proper review as I don't have a way to run elastic atm but this looks like a great step in the right direction.
Once this gets merged it should probably have a beta release at first, to leave room for any breaking changes needed to let this work with services (e.g. algolia) other than elastic.

[madmatt]

Nice one, thanks @chrispenny! Overall it looks good, I agree with you about how index configuration is managed, perhaps I can work through that as part of the Algolia change when I get to it.

[chrispenny]

Manual testing completed in silverstripe/silverstripe-search-service-elastic#1. I'm about as happy as I'm going to get, considering I won't be able to test this on a real project for quite some time.

I'll look to merge this soon, and I'll tag this as a beta.

Thanks for all the help, team :D