CI: macOS-10.15 environment is deprecated (#4664)

* macOS-10.15 environment is deprecated

Build for 10.14 has to be dropped.
Build for 10.15 can be preserved via 11.

* update xcode versions

* Xcode 13.0 doesn't work for us on Big Sur

* [skip ci] update list of binaries

.github/workflows/desktop-build.yml

@@ -201,17 +201,9 @@ jobs:
             type: Debug
             do_tests: 1
 
-          - target: 10.14_Mojave
-            os: macos-10.15 # runs on Catalina
-            xcode: 10.3 # allows compatibility with macOS 10.14
-            qt_version: 5
-            type: Release
-            # do_tests: 1 # tests do not work on qt5?
-            make_package: 1
-
           - target: 10.15_Catalina
-            os: macos-10.15
-            xcode: 12.4
+            os: macos-11
+            xcode: 11.7 # allows using macOS 10.15 SDK
             qt_version: 6
             type: Release
             do_tests: 1
@@ -219,7 +211,7 @@ jobs:
 
           - target: 11_Big_Sur
             os: macos-11
-            xcode: 13.2
+            xcode: 12.5.1
             qt_version: 6
             type: Release
             do_tests: 1