[Tooling] Add Release Publishing automation

[iangmaia]

This PR automates the release publishing, which means to:

• Publish the existing draft release on GitHub (which will also have GitHub create the associated git tag, pointing to the tip of the branch)
• If the release branch for the next version already exists, backmerge the current release into it
• Delete the current release branch

This PR will be followed by updates to the Releases V2 tool.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr13791-4a10dc3
	Version	20.2
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	4a10dc3
	App Center Build	WooCommerce - Prototype Builds #10720

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[AliSoftware]

Same 💭 as woocommerce/woocommerce-android#12450 (comment)

[iangmaia]

I've updated the PR after the release of release-toolkit 12.0.0.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 20.3. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

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