migrate `noarch: python` to CFEP-25's new syntax

Per [CFEP-25](https://github.com/conda-forge/cfep/blob/main/cfep-25.md), `noarch: python` packages should have the syntax

```yaml
host:
- python {{ python_min }}
run:
- python >={{ python_min }}

test:
requires:
- python {{ python_min }}
```

This issue has TODO items for the migration:

- [x] make conda-build robust to variants needed for proper yaml (https://github.com/conda/conda-build/pull/5528)
- [x] update CFEP to use a simpler syntax that is more compatible (https://github.com/conda-forge/cfep/pull/58, https://github.com/conda-forge/conda-smithy/pull/2122, https://github.com/conda-forge/conda-forge.github.io/pull/2360)
- [x] update docs (https://github.com/conda-forge/conda-forge.github.io/pull/2355)
- [x] add `python_min` variable to pinnings (https://github.com/conda-forge/conda-forge-pinning-feedstock/pull/6646)
- [x] write migrator (https://github.com/regro/cf-scripts/pull/3093)
- [x] ~adjust admin command to use new syntax~ (The admin command only adds the parts in build, so I think we should leave it as is. The linter will warn folks and they will have to adjust it by hand.)
- [x] add hints to smithy (https://github.com/conda-forge/conda-smithy/pull/2115, https://github.com/conda-forge/conda-smithy/pull/2119, https://github.com/conda-forge/conda-smithy/pull/2123)
- [x] soften language, improve docs, use jinja2/context (https://github.com/conda-forge/conda-smithy/pull/2126, https://github.com/conda-forge/conda-forge.github.io/pull/2363, https://github.com/conda-forge/conda-forge.github.io/pull/2365)
- [x] fix bug with jinja2 set statement and linter (https://github.com/conda-forge/conda-smithy/pull/2132)
- [x] fix bug in conda-build that causes bad renders (https://github.com/conda/conda-build/pull/5535)
- [x] fix bug in conda-smithy for feedstock creation with `python_min` (https://github.com/conda-forge/conda-smithy/pull/2135, https://github.com/conda-forge/conda-smithy/pull/2144)
- [x] fix bug in conda-build for regression on undefined jinja2 variables (https://github.com/conda/conda-build/pull/5538)
- [x] make announcement (did on zulip)
- [x] fix more v1 bugs (https://github.com/conda-forge/conda-smithy/pull/2184)
- [ ] run migrator to completion