Drop *_key options in favor of new remap option

[binarylogic]

Once #3463 is done, we should drop the various *_key options and allow users to remap events to their liking with the new remap option.

• Preserve backward compatibility by deprecating these options and removing them from our docs.
• ... will fill out with specific options.

[lukesteensen]

This is an interesting idea but I do think it's missing one facet of the *_key options. Those were very much intended to point out the location of well-known fields (e.g. message, host, timestamp) so that downstream components that use those well-known fields would access them correctly. The current implementation happens to accomplish that by mapping them to specific field names, but a more sophisticated implemenation could have simply recorded some metadata.

If we switch to simply using remap for this, it's not clear how downstream components should know which field to read. It can be just convention around the names, but that seems more brittle and error-prone than the metadata-based solution.

[binarylogic]

Closing since this will be handled as part of solving schemas.