Create planning issue for each group

Updates the planning issue creator to create issues for each group, instead of a single issue for all groups, as discussed in https://gitlab.com/gitlab-org/ecosystem-stage/team-tasks/-/issues/76#note_647703633.

Issues that this new code creates (note: managers were not assigned to the issues for the purposes of this MR, to reduce noise. The script still auto-assigns managers):

• Foundations: https://gitlab.com/gitlab-org/ecosystem-stage/team-tasks/-/issues/84 (@tauriedavis, @leipert and @mushakov will be assigned)
• Integrations: https://gitlab.com/gitlab-org/ecosystem-stage/team-tasks/-/issues/83 (@arturoherrero and @mushakov will be assigned)
• Contributor Experience: https://gitlab.com/gitlab-org/ecosystem-stage/team-tasks/-/issues/82 (@leipert and @mushakov will be assigned)