chore: remove "v3" question from issue template (#3017)

python-gitlab hasn't supported the GitLab v3 API since 2018. The last
version of python-gitlab to support it was v1.4

Support was removed in:

commit fe89b949
Author: Gauvain Pocentek <gauvain@pocentek.net>
Date:   Sat May 19 17:10:08 2018 +0200

    Drop API v3 support

    Drop the code, the tests, and update the documentation.