MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

plugin/checker/check_api.go

@@ -8,8 +8,8 @@ import (
 	"go/ast"
 	"go/token"
 
-	"github.com/mattermost/mattermost-server/v5/plugin/checker/internal/asthelpers"
-	"github.com/mattermost/mattermost-server/v5/plugin/checker/internal/version"
+	"github.com/mattermost/mattermost-server/v6/plugin/checker/internal/asthelpers"
+	"github.com/mattermost/mattermost-server/v6/plugin/checker/internal/version"
 )
 
 func checkAPIVersionComments(pkgPath string) (result, error) {

plugin/checker/check_api_test.go

@@ -17,24 +17,24 @@ func TestCheckAPIVersionComments(t *testing.T) {
 	}{
 		{
 			name:    "valid comments",
-			pkgPath: "github.com/mattermost/mattermost-server/v5/plugin/checker/internal/test/valid",
+			pkgPath: "github.com/mattermost/mattermost-server/v6/plugin/checker/internal/test/valid",
 			err:     "",
 		},
 		{
 			name:    "invalid comments",
-			pkgPath: "github.com/mattermost/mattermost-server/v5/plugin/checker/internal/test/invalid",
+			pkgPath: "github.com/mattermost/mattermost-server/v6/plugin/checker/internal/test/invalid",
 			expected: result{
 				Errors: []string{"internal/test/invalid/invalid.go:15:2: missing a minimum server version comment on method InvalidMethod"},
 			},
 		},
 		{
 			name:    "missing API interface",
-			pkgPath: "github.com/mattermost/mattermost-server/v5/plugin/checker/internal/test/missing",
+			pkgPath: "github.com/mattermost/mattermost-server/v6/plugin/checker/internal/test/missing",
 			err:     "could not find API interface",
 		},
 		{
 			name:    "non-existent package path",
-			pkgPath: "github.com/mattermost/mattermost-server/v5/plugin/checker/internal/test/does_not_exist",
+			pkgPath: "github.com/mattermost/mattermost-server/v6/plugin/checker/internal/test/does_not_exist",
 			err:     "could not find API interface",
 		},
 	}

plugin/checker/check_helpers.go

@@ -11,8 +11,8 @@ import (
 
 	"github.com/pkg/errors"
 
-	"github.com/mattermost/mattermost-server/v5/plugin/checker/internal/asthelpers"
-	"github.com/mattermost/mattermost-server/v5/plugin/checker/internal/version"
+	"github.com/mattermost/mattermost-server/v6/plugin/checker/internal/asthelpers"
+	"github.com/mattermost/mattermost-server/v6/plugin/checker/internal/version"
 )
 
 func checkHelpersVersionComments(pkgPath string) (result, error) {

plugin/checker/check_helpers_test.go

@@ -17,12 +17,12 @@ func TestCheckHelpersVersionComments(t *testing.T) {
 	}{
 		{
 			name:     "valid versions",
-			pkgPath:  "github.com/mattermost/mattermost-server/v5/plugin/checker/internal/test/valid",
+			pkgPath:  "github.com/mattermost/mattermost-server/v6/plugin/checker/internal/test/valid",
 			expected: result{},
 		},
 		{
 			name:    "invalid versions",
-			pkgPath: "github.com/mattermost/mattermost-server/v5/plugin/checker/internal/test/invalid",
+			pkgPath: "github.com/mattermost/mattermost-server/v6/plugin/checker/internal/test/invalid",
 			expected: result{
 				Errors:   []string{"internal/test/invalid/invalid.go:20:2: documented minimum server version too low on method LowerVersionMethod"},
 				Warnings: []string{"internal/test/invalid/invalid.go:23:2: documented minimum server version too high on method HigherVersionMethod"},

plugin/checker/main.go

@@ -10,7 +10,7 @@ import (
 	"strings"
 )
 
-const pluginPackagePath = "github.com/mattermost/mattermost-server/v5/plugin"
+const pluginPackagePath = "github.com/mattermost/mattermost-server/v6/plugin"
 
 type result struct {
 	Warnings []string