borg2: hints for maintainers

Some hints for Linux / BSD / other OS package maintainers:

## libs

make sure **ALL** libs have a `LIBNAME.pc` file for `pkg-config` / `pkgconf` because we use that for discovery.

the requirements are documented in our docs / in setup.py.

borg 2.0.0b23 requires openssl >= 1.1.1 now, b24+ will require openssl >= 3.2 (#7963).

## python packages

we'll additionally need:
- up to b23: argon2-cffi (b24+ not needed any more, #7963)
- borghash and borgstore (and requests, optionally rclone and paramiko)
- platformdirs
- backports-zstd (if you package for python < 3.14, 3.14 has zstd in stdlib)
- jsonargparse with shtab extra (also requires PyYAML)
- blake3

For more details, see our `pyproject.toml`.

## python 3.11

we'll require that as a minimum for borg >= 2.0.