Update cmake to gtensor minimum version

[JamesMcClung]

The psc CI/CD recently stopped working (e.g. psc-code/psc#385) because it was using cmake version 3.20.2, whereas gtensor recently updated its cmake requirement to 3.21.

The problem seems to be that Docker is using a cached version of cmake. This repo (psc-infrastructure) was created May 8–14, 2021, which is exactly between the releases of cmake v3.20.2 and v3.20.3 (see here; the page may drift with time), which is consistent with cmake having been cached for the entire history of this repo.

To fix the CI/CD, it would probably suffice to just invalidate the cache (in any number of ways), but I take the approach of pinning the cmake version instead for the sake of clarity/transparency.

As an aside, here are some other links I came across while trying to figure out the CI/CD problem, included for posterity/future reference:

• https://apt.kitware.com/
• https://docs.docker.com/build/cache/invalidation/
• https://stackoverflow.com/questions/35134713/disable-cache-for-specific-run-commands
• caching and “apt-get update” moby/moby#3313

[JamesMcClung]

Also, while I'm pretty sure I identified the problem, I don't know if this is the full solution. TBD, I guess.