chore: replace deprecated String.prototype.substr() (#6333)

* chore: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

* refactor: add prefer slice lint rule and fix errors

Co-authored-by: erezrokah <erezrokah@users.noreply.github.com>

packages/netlify-cms-backend-azure/src/API.ts

@@ -293,7 +293,7 @@ export default class API {
   fromBase64 = (str: string) => Base64.decode(str);
 
   branchToRef = (branch: string): string => `refs/heads/${branch}`;
-  refToBranch = (ref: string): string => ref.substr('refs/heads/'.length);
+  refToBranch = (ref: string): string => ref.slice('refs/heads/'.length);
 
   user = async () => {
     const result = await this.requestJSON<AzureUser>({