Update minimum Go version to 1.21

Required by use of the slices package in test code

Author: bluekeyes

.github/workflows/go.yml

@@ -9,13 +9,13 @@ jobs:
     name: Verify
     runs-on: ubuntu-latest
     steps:
-      - name: Set up Go 1.19
-        uses: actions/setup-go@v3
+      - name: Set up Go 1.21
+        uses: actions/setup-go@v5
         with:
-          go-version: 1.19
+          go-version: 1.21
 
       - name: Check out code into the Go module directory
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Lint
         uses: golangci/golangci-lint-action@v3

go.mod

@@ -1,3 +1,3 @@
 module github.com/bluekeyes/go-gitdiff
 
-go 1.20
+go 1.21