update HOW_TO_RELEASE.md

[jhamman]

This PR updates our step-by-step guide for releasing Xarray. It makes a few minor changes to account for #6206 and officially documents the switch to CALVER. This should be clearly documented in whats-new.rst as part of the first release utilizing CALVER.

Also, note that this should probably wait until we make the 0.20.1 patch release.

• Closes Xarray versioning to switch to CalVer #6176, Remove stable branch? #6206
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[max-sixty]

A few other things we can do:

• Insert things like contributors automatically; I've used fastmod for this sort of automation
  • Not sure whether we can strip out bot names?
• Automate checks for things like "am I on the correct branch", remove checks like "run the tests again" given our CI infra is really good now (thanks @andersy005 & @keewis & others!)
• Automate release notes? Is this possible (without spinning up GPT3 :) )
  • I didn't add a description for them the most recent release
  • We already have whatsnew, maybe that's enough?
  • I rather enjoy reading release notes for libraries I follow! So maybe this is something to spend a few minutes on?

[TomNicholas]

Insert things like contributors automatically; I've used fastmod for this sort of automation
Not sure whether we can strip out bot names?

Yes, let's do that - we could have a bot name red list, but I don't personally think it matters if there are a couple of bot names or duplicated names in the list.

Automate checks for things like "am I on the correct branch", remove checks like "run the tests again" given our CI infra is really good now

If we can do this reliably then yes!

Automate release notes? Is this possible (without spinning up GPT3 :) )
I didn't add a description for them the most recent release
We already have whatsnew, maybe that's enough?
I rather enjoy reading release notes for libraries I follow! So maybe this is something to spend a few minutes on?

I feel like we could just make release notes explicitly optional? So we can write a summary if there is particularly cool feature we want to highlight, but most of the time the whatsnew will do fine?