Release v2.12.0

[bigbes]

This maintenance release marks the end of active development on the v2 branch.

Added

• tt pack: support nested .packignore at the root of tt environment.

Fixed

• tt pack: fix .packignore for directories with regexp symbols in
  their names.

[elhimov]

Nit-1: no need to put release summary into commit message, just title Release vX.Y.Z
Nit-2: task is missing, but it seems reasonable to add exception into check message rules for "release" commit, so let's ignore it for now.

[oleg-jukovec]

Nit-1: no need to put release summary into commit message, just title Release vX.Y.Z