🌱 Add GitHub Action to automate creating a release

[CecileRobertMichon]

What this PR does / why we need it: This is part 1 of automating our release process. It adds a GH action for creating the draft release with release notes and generated artifacts on GitHub when a tag is pushed.

Tested on my fork: https://github.com/CecileRobertMichon/cluster-api/actions/runs/1192045111
Resulting draft release: https://github.com/CecileRobertMichon/cluster-api/releases/tag/untagged-88e1db0cbb70a7f9fe19

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 #

[CecileRobertMichon]

Testing on my fork: https://github.com/CecileRobertMichon/cluster-api/actions/runs/1192045111
Resulting draft release: https://github.com/CecileRobertMichon/cluster-api/releases/tag/untagged-88e1db0cbb70a7f9fe19

still running
/hold
until it succeeds

[CecileRobertMichon]

/hold cancel

verified this works

[CecileRobertMichon]

/assign @vincepri

[vincepri]

@CecileRobertMichon This is amazing ❤️ thank you so much for doing this!

Do you have a screenshot of the resulting draft on your fork? The link doesn't seem to work for me

[sbueringer]

Do you have a screenshot of the resulting draft on your fork? The link doesn't seem to work for me

I assume that's because draft release are not published and thus private :)

Very nice!

[CecileRobertMichon]

The release notes are pretty long but here are screenshots of the beginning and the end:

(it's using an old tag I had on my fork as the previous tag)

[vincepri]

This is great!

[CecileRobertMichon]

/hold cancel

latest run: https://github.com/CecileRobertMichon/cluster-api/runs/3499020825
resulting release:

[CecileRobertMichon]

re-testing with latest changes: https://github.com/CecileRobertMichon/cluster-api/actions/runs/1195651990

[vincepri]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vincepri

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[CecileRobertMichon]

/cherry-pick release-0.3

[k8s-infra-cherrypick-robot]

@CecileRobertMichon: #5188 failed to apply on top of branch "release-0.3":

Applying: seedling: Add GitHub Action to automate creating a release
Using index info to reconstruct a base tree...
M	.gitignore
M	Makefile
M	docs/developer/releasing.md
M	test/infrastructure/docker/Makefile
Falling back to patching base and 3-way merge...
Auto-merging test/infrastructure/docker/Makefile
CONFLICT (content): Merge conflict in test/infrastructure/docker/Makefile
Auto-merging docs/developer/releasing.md
CONFLICT (content): Merge conflict in docs/developer/releasing.md
Auto-merging Makefile
CONFLICT (content): Merge conflict in Makefile
Auto-merging .gitignore
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 seedling: Add GitHub Action to automate creating a release
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release-0.3

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