Update testing/compatibility matrices: Drop Drupal 10.1, add 10.4.x-dev.

[rosiel]

Drupal 10.3 has been released!

This is a parent ticket for a number of things we said we were going to do:

• Stop testing on 10.1.x
• Change the testing matrix from 10.3.x-dev to 10.3.x
• Add 10.4.x to the testing matrix.
• Remove any compatibility code that's for 10.1, and if necessary, add a constraint that the code requires 10.2 or above.

This is not addressing the more work that needs to be done to get to D11 compatibility: #2318