Deprecate method instead of deleting it

Preserve API compatibility for consumers.  Low risk to delete it, but
it is simpler for maintainers if we retain the method rather than answer
later questions about its removal.

Author: MarkEWaite

src/main/java/hudson/plugins/git/GitSCM.java

@@ -1810,6 +1810,17 @@ public void setAllowSecondFetch(boolean allowSecondFetch) {
 
         public void setAddGitTagAction(boolean addGitTagAction) { this.addGitTagAction = addGitTagAction; }
 
+        /**
+         * Old configuration of git executable, unused since 2023.
+         * Returns null in all cases.
+         * @deprecated use GitTool
+         * @return null
+         */
+        @Deprecated(since = "5.11.0")
+        public String getOldGitExe() {
+            return null;
+        }
+
         public static List<RemoteConfig> createRepositoryConfigurations(String[] urls,
                 String[] repoNames,
                 String[] refs) throws GitException, IOException {