Migrate gitian builds to Debian Bullseye

Summary:
This is step toward migrating our build infra from Debian 10 Buster (now
obsolet) to Debian 11 Bullseye. The sensible part here is the use of
gcc/g++ version 9 instead of the default version 10. It prevents from
using the new symbol pthread_cond_clockwait introduced in glibc 2.30
and would make our release incompatible with previous glibc versions.
I didn't manage to prevent using this symbol during the compilation with
gcc/g++ 10, so I'm moving ahead with version 9.

Test Plan:
Run each gitian build twice. Check the hashes are deterministic. Check
the binaries run fine on each platform.

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12103