Add Clang 11 in settings.yml

[uilianries]

Changelog: Feature: Supports Clang 11.
Docs: conan-io/docs#1883

closes #7223

Clang 10 still uses C++14 by default and same flag for c++20 (c++2a). Thus, _cppstd_clang is totally fine.

https://releases.llvm.org/10.0.0/tools/clang/docs/ReleaseNotes.html
https://releases.llvm.org/10.0.0/tools/clang/docs/index.html

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[uilianries]

It would be great if we could use Jinja for migrations_settings.py, that file has 1k8 lines!

[memsharded]

It would be great if we could use Jinja for migrations_settings.py, that file has 1k8 lines!

This will be removed in Conan 2.0