Move EE-specific migrations under ee/db/

What does this MR do?

This moves EE-specific migrations under ee/db/.

I've used the following command to move the migrations files:

$ mkdir -p ee/db/{,post_,geo/}migrate
$ git diff ce/master..origin/master --name-status -- db/{geo/migrate,migrate,post_migrate} | grep A | cut -f 2 | xargs -t -L1 -I % mv % ee/%

Are there points in the code the reviewer needs to double check?

I've also moved db/geo under ee/db/geo and updated the code/docs that were relying on the previous location.

Why was this MR needed?

To separate EE code from CE code.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3094 (closed)