Bump debhelper compat to 13

This fixes some Lintian pedantic warnings.

I'm setting as draft because debhelper compat 13 enables dh_missing --fail-missing by default and I don't know which binary package (libjxl-devtools or libjxl-tools) should contain these:

usr/bin/exr_to_pq
usr/bin/local_tone_map
usr/bin/ssimulacra2

(It's possible to override dh_missing but I assume we want to install them.)

I guess you could add manpages for them too if you want.

marked this merge request as ready

merged

mentioned in commit 96bba226