resource/gitlab_project: Mark `suggestion_commit_message` as computed

Timo Furrerrequested to merge
bugfix/suggest-commit-msg-optional into main

The suggestion_commit_message attribute which was introduced in !1249 (merged) and released with %v3.19.0 wasn't marked as Computed: true which can cause unexpected Note: Objects have changed outside of Terraform messages when applying.

/cc @nolith

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