Keep thunderbird 91 in impish-proposed until a point release enables profile upgrades

[Olivier Tilloy]

Thunderbird 91 was released on August 11. According to the release notes (https://www.thunderbird.net/en-US/thunderbird/91.0/releasenotes/):

  « Thunderbird version 91.0 is only offered as direct download from thunderbird.net and not as an upgrade from Thunderbird version 78 or earlier. A future release will provide updates from earlier versions. »

To follow upstream's lead on safe upgrades, let's keep 91 in impish-proposed for now, until a later point release enables upgrades, hopefully before final freeze.

[Olivier Tilloy (osomon)]

The latest stable release of thunderbird is 91.1.1 (https://www.thunderbird.net/en-US/thunderbird/91.1.1/releasenotes/), and it is available in impish-proposed.

Upstream hasn't flipped the switch to enable upgrades from 78 to 91 yet, but in my limited testing there is nothing preventing/breaking the upgrade path. I have tested that a profile created in thunderbird 78 is correctly re-used after upgrading to 91 in impish-proposed.

Given that 78.15.0 (to be released on October 5) will be the last release of the 78 branch (after which it will become unsupported) and that impish is not a LTS, I think we should unblock thunderbird 91 from proposed. This will:

 - get it some wider field testing

 - ensure that we don't release impish with a series of thunderbird that is EOL

[Francisco Pombal (pombal.francisco)]

Thumbs up to go ahead with 91 on impish.

Indeed, it does not seem like a great idea to:

- release impish with a series of thunderbird that is EOL (as @osomon stated)
- go into the 22.04 cycle without having proven tb >= 91 in at least one previous release

Furthermore, as of this writing, the disclaimer concerning profile upgrades is no longer present in the release notes for 91.1.2, as opposed to previous 91.x releases.

I would bet that upstream is confident that everything is in order, but it is also possible that they simply forgot to add the disclaimer thus far - here is an archive link as proof anyway: https://web.archive.org/web/20210929002603/https://www.thunderbird.net/en-US/thunderbird/91.1.2/releasenotes/