According to https://endoflife.software/programming-languages/server-side-scripting/ruby
ruby 2.4 will go end-of-life in march, where the new release of nixpkgs
will be cut. We won't be able to support it for security updates.

Remove all references to ruby_2_4 and add ruby_2_7 instead where
missing.

Mark packages that depend on ruby 2.4 as broken:
* chefdk
* sonic-pi

(cherry picked from commit bcdc90a)

(cherry picked from commit 48603cd)

(cherry picked from commit 82d9ce4)

(cherry picked from commit 187d691)

(cherry picked from commit 8019df9)

(cherry picked from commit 1192073)

(cherry picked from commit f43fdd1)

…rsion

Also adds this to the release jobset.

(cherry picked from commit 7a625e7)

(cherry picked from commit 4ff2a16)

[20.03] firefox 73

This should improve the speed of bootstrapping process.
Cost of evaluation also decreases a bit,
but I don't expect that will be significant.

(cherry picked from commit f651910)

Thanks to python3Minimal.  This reverts part of c203848 #79738.

(cherry picked from commit 5a8000d)