Not allow # in index template name (elastic#195776)

Closes [elastic#81870](elastic#81870)

## Summary

When creating a new Template, it did not prevent entering a #, but when
saving the template the user received an error. The hash has been added
to the list of invalid characters for the name.

https://github.com/user-attachments/assets/2b59d245-c96f-4215-ad89-a3201bef5e94
(cherry picked from commit 4339f84)

x-pack/plugins/index_management/common/constants/invalid_characters.ts

@@ -7,4 +7,4 @@
 
 export const INVALID_INDEX_PATTERN_CHARS = ['\\', '/', '?', '"', '<', '>', '|'];
 
-export const INVALID_TEMPLATE_NAME_CHARS = ['"', '*', '\\', ',', '?'];
+export const INVALID_TEMPLATE_NAME_CHARS = ['"', '*', '\\', ',', '?', '#'];