New release automations

[t3chguy]

Fixes https://github.com/vector-im/wat-internal/issues/21

Notes:

• Changes our first RC from rc.1 to rc.0
• Switches from allchange to release-drafter set up with similar rules

Example:

t3chguy#21 before RC:

Finalise RC0:

t3chguy#22 after RC:

Finalise as final:

Workflow:

• Release captain uses automation to cut branches
• Release captain waits for draft release to be made and reads the release notes for sanity and makes any changes necessary, including changing version number
• Release captain uses automation to promote the draft release into an RC 0-N times as appropriate
• Release captain uses automation to promote the draft release into a final one

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This change is marked as an internal change (Task), so will not be included in the changelog.