[doc] Bump minimum GCC dependency

Summary:
GCC 10.x does not build for me due to lack of support of
std::source_location. Considering it first appears as no longer
experimental in GCC 12 docs, I suspect it will not work in GCC 11
despite Core saying they support GCC 11 here:
https://github.com/bitcoin/bitcoin/pull/29091/

Debian Bookworm (12) packages GCC 12 by default so this makes sense
as the minimum supported version for ABC.

Test Plan:

• build with gcc 10.x (fails)
• build with gcc 12.x (succeeds)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18911