Remove version numbers in macos triplets

### Your question:

Currently our default triplets are
- x86_64-apple-darwin13.4.0 (corresponding to 10.9 even though our default is 10.13 now)
- arm64-apple-darwin20.0.0 (corresponding to 11.0)

We can drop the version numbers, but AFAIR some autoconf/libtool scripts checked `*darwin[12][0-9]*` or something similar for detecting features. Not sure if this is relevant today.

---

Some previous context (added by @h-vetinari):

This came up in https://github.com/conda-forge/conda-forge-pinning-feedstock/pull/5829 in the context of moving from 10.9 -> 10.13 (which would strictly speaking cause the darwin version to increase), but decided to keep the version in the target name at the time, and postpone the removal to some later date (i.e. what this issue is about). At the time, Isuru wrote
> If there's a triplet that works without macos version, that'd be great, but autoconf has some checks for the darwin kernel for existence of `dynamic_lookup` option if I remember correctly.

It also came up in https://github.com/conda-forge/ctng-compilers-feedstock/pull/189 / https://github.com/conda-forge/clangdev-feedstock/pull/405 more recently. Aside from our toolchain-related feedstocks, this value isn't being set in [many places](https://github.com/search?q=org%3Aconda-forge%20%2Fapple-darwin%5Cd%2F%20path%3Arecipe&type=code) within conda-forge.