Allow installation on Group 4.x

Problem/Motivation

group_action 1.2.2 declares "drupal/group": "^1 || ^2@beta || ^3@beta" in its
composer.json, which blocks installing it alongside Group 4.x —
Composer refuses to resolve.

The runtime code is already Group 4.x compatible. GroupActionBase resolves a static $isV2 flag
(documented "v2 or greater") by checking whether the container has the group_relation_type.manager service — present in
Group 2, 3 and 4. On Group 4.x the flag is TRUE, so every Group call routes through the $isV2-true branch,
which uses only the stable getRelationshipsByEntity() and addRelationship() APIs. The removed
getContent() / addContent() calls exist only in the unreachable v1 branch.

Proposed resolution

Widen the drupal/group constraint in composer.json to include ^4:

"drupal/group": "^1 || ^2@beta || ^3@beta || ^4"

An MR follows on the issue fork.