Run 'go fix' to modernize code (#69)

This only updates to Go 1.21 baseline, which is what the module still
specifies as the minimum version. I'll need to run this again when
moving to Go 1.25 or 1.26 as minimum.

gitdiff/patch_header_test.go

@@ -11,7 +11,7 @@ func TestParsePatchDate(t *testing.T) {
 	tests := map[string]struct {
 		Input  string
 		Output time.Time
-		Err    interface{}
+		Err    any
 	}{
 		"default": {
 			Input:  "Thu Apr 9 01:07:06 2020 -0700",
@@ -88,7 +88,7 @@ func TestParsePatchHeader(t *testing.T) {
 		Input   string
 		Options []PatchHeaderOption
 		Header  PatchHeader
-		Err     interface{}
+		Err     any
 	}{
 		"prettyShort": {
 			Input: `commit 61f5cd90bed4d204ee3feb3aa41ee91d4734855b