### What problem does this PR solve?
Currently singing up to ragflow using a mail-adress with associated
top-level domains that have more than 4 chars will fail due to a regex
validation that enforces just this.
In our use case, we'd like to use e-mail addresses with `.swiss`
top-level domains, which is a valid TLD associated with the country
switzerland in the IANA root database.
This change makes the validation accept 5-letter TLDs.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Other (please describe): Making validation for lenient, accepting
more valid input.
6850fd69c6