Update CMake version

[etcwilde]

LLVM requires at least CMake 3.20 on next. Bumping the CMake version to unblock work.

CMake Error at CMakeLists.txt:3 (cmake_minimum_required):
  CMake 3.20.0 or higher is required.  You are running version 3.19.6

[etcwilde]

@swift-ci please test

[etcwilde]

Made it past the CMake version failure and into issues in LLVM that are fixed in https://reviews.llvm.org/D154664.
(though half of the below is also due to a leaky architecture)

/Users/ec2-user/jenkins/workspace/swift-PR-macos@2/branch-next/llvm-project/llvm/include/llvm/Support/Format.h:220:3: error: no template named 'optional' in namespace 'std'; did you mean 'Optional'?
  220 |   std::optional<uint64_t> FirstByteOffset;
      |   ^~~~~~~~~~~~~
      |   Optional
/Users/ec2-user/jenkins/workspace/swift-PR-macos@2/branch-next/swift/stdlib/include/llvm/ADT/Optional.h:251:29: note: 'Optional' declared here
  251 | template <typename T> class Optional {
      |                             ^

[edymtt]

The plan I discussed offline with @shahmishal was to bump to CMake 3.24.2 in main, and have that change propagate to next -- #67018

[shahmishal]

We can close this PR, I have merged #67018