Up Minimum CMake Version to 3.20 (#86530)

* Let's see what happens :)

* CMake 3.20 works on my machine???

* Upped the llibraries CMake version to 3.20

* Upped native libs CMake version to 3.20, and updated the requirements docs.

* Add CMake Policy to 3.17.0 for native libs because of wasm.

* Downgraded native libs CMake to 3.17.0 temporarily, while we figure out the funny troublemaker wasm.

* Added a couple notes regarding CMake on Linux and Windows, and upped the corehost and libunwind CMake versions to 3.20

* Reverted external native components CMake versions, and upped tests to 3.20. Only Mono remains now.

* Added mono to the 3.20 gang. Also, changed the condition for wasm.

* Had to condition mono's CMake version as well because of wasm.

* DEBUG-ONLY COMMIT: Added some CMake logging to help me figure out what is set for wasm to think of the appropriate conditional.

* DEBUG-ONLY COMMIT: I think I have the answer for wasm now/

* Restored the DEBUG-ONLY changes and set 3.20 as the universal CMake, as the missing machines have now been updated.

* Updated requirements doc with notes for Debian 11 and lower.

docs/workflow/requirements/linux-requirements.md

@@ -24,7 +24,7 @@ Minimum RAM required to build is 1GB. The build is known to fail on 512 MB VMs (
 
 Install the following packages for the toolchain:
 
-* CMake 3.14.5 or newer
+* CMake 3.20 or newer
 * llvm
 * lld
 * clang
@@ -40,12 +40,24 @@ Install the following packages for the toolchain:
 * zlib1g-dev
 * ninja-build (optional, enables building native code with ninja instead of make)
 
+**NOTE**: If you have an Ubuntu version older than 22.04 LTS, or Debian version older than 12, don't install `cmake` using `apt` directly. Follow the note written down below.
+
 ```bash
 sudo apt install -y cmake llvm lld clang build-essential \
 python-is-python3 curl git lldb libicu-dev liblttng-ust-dev \
 libssl-dev libkrb5-dev zlib1g-dev ninja-build
 ```
 
+**NOTE**: As of now, Ubuntu's `apt` only has until CMake version 3.16.3 if you're using Ubuntu 20.04 LTS (less in older Ubuntu versions), and version 3.18.4 in Debian 11 (less in older Debian versions). This is lower than the required 3.20, which in turn makes it incompatible with the repo. For this case, we can use the `snap` package manager or the _Kitware APT feed_ to get a new enough version of CMake.
+
+For snap:
+
+```bash
+sudo snap install cmake
+```
+
+For the _Kitware APT feed_, follow its [instructions here](https://apt.kitware.com/).
+
 You now have all the required components.
 
 #### Additional Requirements for Cross-Building

docs/workflow/requirements/macos-requirements.md

@@ -23,7 +23,7 @@ Building _dotnet/runtime_ depends on several tools to be installed. You can down
 
 Install the following packages:
 
-* CMake 3.15.5 or newer
+* CMake 3.20 or newer
 * icu4c
 * openssl@1.1 or openssl@3
 * pkg-config

docs/workflow/requirements/windows-requirements.md

@@ -51,7 +51,9 @@ These steps are required only in case the tools have not been installed as Visua
 * Install [CMake](https://cmake.org/download) for Windows.
 * Add its location (e.g. C:\Program Files (x86)\CMake\bin) to the PATH environment variable. The installation script has a check box to do this, but you can do it yourself after the fact following the instructions at [Adding to the Default PATH variable](#adding-to-the-default-path-variable).
 
-The _dotnet/runtime_ repository recommends using CMake 3.16.4 or newer, but it may work with CMake 3.15.5.
+The _dotnet/runtime_ repository requires using CMake 3.20 or newer.
+
+**NOTE**: If you plan on using the `-msbuild` flag for building the repo, you will need version 3.21 at least. This is because the VS2022 generator doesn't exist in CMake until said version.
 
 #### Ninja