Two upgrade-related blockers need review

[Jeroen Demeyer]

In order to fix upgrading, there are two tickets which need review:

#13405: Remove old docs when upgrading sagenb: When upgrading the
notebook from the pre-flask notebook, the reference manual gives
WARNINGs because of old .rst files still present.

#13395: Fix upgrading with GCC: The GCC spkg broke upgrading from old
Sage versions because of the LD_LIBRARY_PATH not containing lib32 or
lib64. We can fix this using Leif's idea of sage-env versioning (see
#10469). We should also rebuild GCC if gcc was already installed before.


I made these blockers because I think it's good to fix upgrading.