CDRIVER-4580 announce plan to drop macOS 10.14 and 10.15 (#1217)

* announce plan to drop macOS 10.14 and 10.15

* note testing with macOS 10.14 will be dropped

* document macOS 11.0 is tested

* document both x86_64 and arm64 are tested

* add TODO comment to drop testing of macOS 10.14 after release of 1.24.0

* refer to macOS 11.0 in "How to Ask For Help"

* refer to "Supported Platforms" in CONTRIBUTING.md

* use "upcoming" in NEWS

This does not commit to dropping in 1.25.0 in case plans change.

* remove mention of macOS 10.15

Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>

* remove comment

* Use "an"

Co-authored-by: Roberto C. Sánchez <roberto@connexer.com>

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Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>
Co-authored-by: Roberto C. Sánchez <roberto@connexer.com>