Stop using GitHub projects

[jonmeow]

I think GitHub's new project are necessary to get contributions going correctly (it seems v1 ACLs don't work right for triage). Notes:

• Add v2 projects to org as "xyz v2" or similar
• Mirror columns over
• Add v2 projects to repo
• Mirror tasks over
• Disable (or immediately replace) proposal project actions
• Remove v1 projects
• Rename v2 projects to v1 names
• Figure out new proposal project actions
• Update new_proposal.py to set v2 project

[jonmeow]

After talking with chandlerc, we decided to disassemble the project boards in favor of only having labels to track. The problem with GitHub is that projects v1 is clearly on its way out and we can't grant access separate from repo write access, while projects v2 isn't ready yet (lacks events to trigger on). So the thought is labels with maybe some action helpers is the right path forward, and using projects was a misstep based on the support level.

#1981 does this for proposals. Note we already had information in labels, it's just that the project had been getting treated as the source of record, and now labels will now be the only source. Proposals will generally have two labels: "proposal" to easily search them all, and "proposal " (e.g. "proposal rfc") to designate their state.

#1995 describes this for leads questions:

"Blocking issues" -> gone, we haven't been using it
"Questions" -> "leads questions" label, open: https://github.com/carbon-language/carbon-lang/issues?
q=is%3Aissue+is%3Aopen+label%3A%22leads+question%22
"Resolved" -> "leads questions" label, closed: https://github.com/carbon-language/carbon-lang/issues?q=is%3Aissue+is%3Aclosed+label%3A%22leads+question%22+
"Needs proposal" -> file a tracking issue, remark on the linked issue
"Deferred" -> add the "long term" label: https://github.com/carbon-language/carbon-lang/issues?q=is%3Aissue+is%3Aopen+label%3A%22leads+question%22+label%3A%22long+term%22

[jonmeow]

Had some permissions issues I figured out after a few tries (#2012 #2013 #2014), and I think this is now done.

[jonmeow]

Also #2019