Add Xcode 12

[ryaneverett33]

Changelog: Feature: Add MacOS 11 ("Big Sur") support.
Docs: conan-io/docs#1819

Found in #7286 but doesn't wholly close out that issue, only adds Xcode 12 as a recognized compiler for apple-clang.

closes #7635

[CLAassistant]

All committers have signed the CLA.

[memsharded]

According to https://gist.github.com/yamaya/2924292, and https://en.wikipedia.org/wiki/Xcode#12.x_series this version is still not released? Is it still in beta stage, right?

We might want to wait until it is officially released. Specially if there is some risk that this version will change some default, like the cppstdp

[ryaneverett33]

Yeah Xcode 12 is still in the beta stage (beta 6 currently) so I would totally understand if it this PR needs to wait. One complication to that worth considering is the DTKs as you have to use Xcode 12 for support for macOS on ARM. However, since we're already quite far into the beta cycle, the public Xcode 12 release might not be too far away so it might not be bad to just wait for that.
Either way, thanks for looking at this PR!

[tpruzina]

Would you mind also adding BigSur ("11.0") to Macos section?

version: [None, "10.6", "10.7", "10.8", "10.9", "10.10", "10.11", "10.12", "10.13", "10.14", "10.15", "11.0"]

[SSE4]

@tpruzina yes, it should be definitely added

[ryaneverett33]

@tpruzina Thanks for mentioning Big Sur; added the version numbers with the last commit.

[czoido]

Hi @Changer098,
We had to release 1.29.2 quickly and I overlooked this PR, sorry. But as the support for Big Sur is not already merged I'm pushing some changes to this PR to add it to the 1.30 migration settings so that you can count as a contributor and it will be merged for 1.30 release.
Thanks a lot for the contribution!