After grace period, deactivate old endpoint and update main branch/deploy logic

[eloquence]

Tor Browser 10.5 was released today (https://blog.torproject.org/new-release-tor-browser-105). It ships the new 2021 endpoint https://securedrop.org/https-everywhere-2021/ as the default prefix for the HTTPSEverywhere ruleset.

After a suitable grace period, we should:

• remove the old endpoint https://securedrop.org/https-everywhere/, since it will no longer be updated
• promote the key-rotation-2021 branch to be the new main branch
• update the deploy logic to use main for https://securedrop.org/https-everywhere-2021/

The Tails 4.20 release (2021-07-13) seems like the earliest opportunity for making these changes, provided that it ships with Tor Browser 10.5. Note that this will likely still introduce a limited amount of breakage for users who have not updated their Tails release yet, but auto-updates should cover most users.

[eloquence]

We are aiming to retire the old endpoint approximately one week after the release of Tails 4.20, assuming it does indeed ship with Tor Browser 10.5.

[eloquence]

Just to confirm, Tails 4.20 does indeed ship with Tor Browser 10.5.2, and the new ruleset endpoint.

[eloquence]

This is mostly done, as far as I can tell the last remaining step is to schedule a time for a careful merge of #66, which will switch main to the branch signed with the new key, and should trigger a deploy (we need to be able to quickly revert in case of any issues).

[eloquence]

This was resolved via #66