Improve the dotnet version checks in configure.ac

- The minimum version should have been updated to 7.0 to go along with PR #595

- Use the target framework for whichever newer dotnet version is found, instead of hardcoding only net8.0. This allows for more forward compatibility if more dotnet versions are released before a newer Pinta release

Fixes: #616

configure.ac

@@ -14,17 +14,16 @@ if test "x$DOTNET" = "xno"; then
 fi
 
 # Check dotnet version
-DOTNET_MINIMUM_VERSION=6.0
+DOTNET_MINIMUM_VERSION=7.0
 DOTNET_VERSION=$($DOTNET --version)
 AX_COMPARE_VERSION([$DOTNET_VERSION], [ge], [$DOTNET_MINIMUM_VERSION],
     AC_MSG_NOTICE([Found dotnet $DOTNET_VERSION]),
     AC_MSG_ERROR([dotnet $DOTNET_MINIMUM_VERSION or higher is required (found version $DOTNET_VERSION).])
 )
 
 # Select target framework based on the dotnet version.
-DOTNET_TARGET_FRAMEWORK=net7.0
-AX_COMPARE_VERSION([$DOTNET_VERSION], [ge], [8.0],
-                   [DOTNET_TARGET_FRAMEWORK=net8.0], [])
+DOTNET_MAJOR_VERSION=$(echo $DOTNET_VERSION | cut -d '.' -f 1,2)
+DOTNET_TARGET_FRAMEWORK="net$DOTNET_MAJOR_VERSION"
 AC_MSG_NOTICE([Using target framework $DOTNET_TARGET_FRAMEWORK])
 
 DOTNET_CMD="env TargetFramework=$DOTNET_TARGET_FRAMEWORK $DOTNET"