TPA-RFC-11: SVN retirement

Draft a proposal to retire SVN altogether. This was somewhat agreed on in #17202, and there are discussions on how exactly to do this in #32273 (how to archive it) and #32025 (how stop corpsvn specifically, the remaining live repo), but this was all done before we came up with the TPA-RFC process.

Now that we have that process, it seems logical to go through with it explicitly so that all stakeholders can express their concerns about the change. I specifically plan on having a live call with sue about this, since she's the most impacted by this.

I create this ticket to track that proposal because #17202 has been sitting in the icebox forever and it's kind of hard to "grab" because it feels so big. Having "write a proposal" first seems more accessible.

draft proposal is in https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-11-svn-retirement

Next steps:

• @susan draft requirements for the sensitive file storage service
• @anarcat research whether we can restrict external sharing in Nextcloud
• @anarcat schedule a meeting in late june to revise the above