Migrate sourcesecret package to runtime/secrets APIs

The sourcesecret package now uses pkg/runtime/secrets factory
functions instead of the previous monolithic approach. This
provides standardized secret generation with consistent
validation and error handling across all authentication types.

Signed-off-by: cappyzawa <cappyzawa@gmail.com>

cmd/flux/create_source_git.go

@@ -305,7 +305,7 @@ func createSourceGitCmdRun(cmd *cobra.Command, args []string) error {
 			secretOpts.Username = sourceGitArgs.username
 			secretOpts.Password = sourceGitArgs.password
 		}
-		secret, err := sourcesecret.Generate(secretOpts)
+		secret, err := sourcesecret.GenerateGit(secretOpts)
 		if err != nil {
 			return err
 		}