drop numpy 1.21 per NEP29

[ngam]

Following @h-vetinari in #4214

It's NEP29 time again, which says

On Jan 31, 2023 drop support for NumPy 1.20 (initially released on Jan 31, 2021)
Previous PRs along these lines: #3420 #2600

NEP29: https://numpy.org/neps/nep-0029-deprecation_policy.html

On Jun 23, 2023 drop support for NumPy 1.21 (initially released on Jun 22, 2021)

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[h-vetinari]

LGTM, thanks!

I'm not 100% sure we want to formulate the policy so strongly, but it's been working fine for a while already, so I'm not against it. I'd suggest to soften the formulation a bit to give ourselves a little wiggle room.

[hmaarrfk]

$ mamba install --yes nep29 && nep29 numpy
| version |    date    |
|:-------:|:----------:|
|  1.25.0 | 2023-06-17 |
|  1.24.0 | 2022-12-18 |
|  1.23.0 | 2022-06-22 |
|  1.22.0 | 2021-12-31 |

[h-vetinari]

This should be uncontroversial (we've been following NEP29 w.r.t. to numpy with small delays for ~2 years already), so in it goes. Thanks @ngam!