Bump minimum supported MacOS version

[milljm]

Motivation

There is a requirement in MOOSE which raises the bar as to the minimum supported version of MacOS:

moose/framework/contrib/tinyhttp/src/tinyhttp/http.C:27:37: error: use of undeclared identifier 'MSG_NOSIGNAL'
    if (::send(mSocket, what, size, MSG_NOSIGNAL) < 0)

This will require a change to our conda_build_config.yaml file(s) to move from SDK 10.15, to an 11.x.

Design

Remove 10.15 SDK, and allow all Mac targets to require a minimum of 11.3.

 #### Darwin SDK SYSROOT
 CONDA_BUILD_SYSROOT:                                        # [osx]
-  - /opt/MacOSX10.15.sdk                                    # [not arm64 and osx]
-  - /opt/MacOSX11.3.sdk                                     # [arm64]
+  - /opt/MacOSX11.3.sdk                                     # [osx]
 
 macos_min_version:                                          # [osx]
-  - 10.15                                                   # [not arm64 and osx]
-  - 11.3                                                    # [arm64]
+  - 11.3                                                    # [osx]
 
 macos_machine:                                              # [osx]
   - x86_64-apple-darwin13.4.0                               # [not arm64 and osx]
   - arm64-apple-darwin20.0.0                                # [arm64]
 
 MACOSX_DEPLOYMENT_TARGET:                                   # [osx]
-  - 10.15                                                   # [not arm64 and osx]
-  - 11.3                                                    # [arm64]
+  - 11.3                                                    # [osx]

Impact

Minimum support MacOS version will go from Catalina to Big Sur.