Update project Go version to 1.21.0

Update minimum Go version to reflect dependency updates.

go.mod

@@ -16,7 +16,7 @@ module github.com/atc0005/elbow
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-go 1.19
+go 1.21.0
 
 require (
 	github.com/alexflint/go-arg v1.5.1