📖 Add issue template for release tasks

[lentzi90]

What this PR does / why we need it:

This adds an issue template for tracking release activities, and updates the release docs with post-release tasks.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #2569

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• [x] squashed commits
• if necessary:
  • [x] includes documentation
  • [ ] adds unit tests

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[lentzi90]

@mdbooth @EmilienM what do you think about this? Is it too much to have the issue template? It does introduce some duplication, but I think it may be a useful way to keep track of release work.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mdbooth

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[lentzi90]

/test pull-cluster-api-provider-openstack-e2e-test