Chore: Replace deprecated String.prototype.substr() (#46763)

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

packages/grafana-toolkit/src/cli/utils/githubRelease.ts

@@ -8,7 +8,7 @@ import { AxiosResponse } from 'axios';
 
 const resolveContentType = (extension: string): string => {
   if (extension.startsWith('.')) {
-    extension = extension.substr(1);
+    extension = extension.slice(1);
   }
   switch (extension) {
     case 'zip':