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Additionally, clean up all Debian Stretch references.

This is useful for Tails' QA when integrating new Tor Browser major
releases.

Thanks cypherpunk

Reported by a cypherpunk, thanks!

Found by arma.

Versions bump and Changelog update

ESR 78 compiled with GCC 9.3.0 requires GLIBCXX_3.4.28.

Instead we generate the string from the version.

Mozilla fixed the regression in bug 1651680.

app.update.url does not exist anymore:
https://bugzilla.mozilla.org/show_bug.cgi?id=1568994

This updates Snowflake and the set of default stun servers used by the
Snowflake client.

At the same time we remove the MBSDIFF environment variable as it is not
needed for full .mar file generation.