Update default merge request template: backward (in)compatible changes

The default merge request template should be updated to the following, which makes it clearer what changes are considered backward (in)compatible:

#### Backwards Compatible Changes

- [ ] This change does not modify a public C header file
- [ ] This change adds new functions/structs/types to a public C header file

#### Backwards Incompatible Changes

- [ ] This change modifies an existing function/struct/type in a public C header file
- [ ] This change removes an existing function/struct/type in a public C header file