Fix: Integrity constraint violation: 1052 Column 'id' in where clause is ambiguous

[iPurpl3x]

Changes proposed in this pull request:
Analog to #3696, changed 'id' to 'groups.id' to avoid Column 'id' in where clause is ambiguous error.

This error normally doesn't occur, but if an extension adds an id column to the group_user table, then this error would occur.

(Adding that column helps when one wants to chunkById() on the group_user model)

Reviewers should focus on:
Nothing is broken by this minor change.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.