[doc] Cleanup minimum supported Debian versions

Summary:
Security support for Bullseye (11) ended over a year ago. While LTS still has 9 months, Bullseye doesn't provide minimum dependencies necessary to build ABC anymore. CI has been stable on Bookworm (12) for 5 months.

This patch cleans up references to old Debian versions, makes some comments more robust to version changes, and updates others where appropriate.

Test Plan: read

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18910