Added in version 3.30.
Specify the default value for the file(DOWNLOAD) and
file(UPLOAD) commands' TLS_VERSION option.
If this variable is not set, the commands check the
CMAKE_TLS_VERSION environment variable.
The value may be one of:
1.0
1.1
1.2
1.3
This variable is also used by the ExternalProject and
FetchContent modules for internal calls to
file(DOWNLOAD) and git clone.