GitHub Actions: update to macos-latest OS

`macos-13` has been deprecated and is no longer available. Since we've
since given up the .NET Core 2/3 TFMs it seems we can now safely update.

References:
actions/runner-images#13046

Author: stakx

.github/workflows/build.yml

@@ -9,14 +9,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ ubuntu-latest, windows-latest, macos-13 ]
-
-        # NOTE: If we wanted to use `macos-latest` we would have to move the .NET Core 2.1 and 3.1 builds and test runs
-        # to a separate job. This is because `macos-14` and newer are ARM-only and those target frameworks don't support
-        # that architecture, causing `dotnet` to want to fall back to X64. However, once we install .NET 6 or newer,
-        # we get a toolchain that only has ARM support and no X64 support, so that fallback will no longer work.
-        # Using `macos-13` is (for the time being, while still available) the simpler solution as it is not ARM-based yet,
-        # so there won't be any architecture mismatch in the first place.
+        os: [ ubuntu-latest, windows-latest, macos-latest ]
 
     runs-on: ${{ matrix.os }}