Modify regex pattern for Angolan phone numbers

[renaldodev]

The regex pattern for Angolan phone numbers was updated to allow for more flexible matching of valid phone numbers.
This modification introduces two changes:
Optional + sign: The phone number is now optionally allowed to begin with the + sign (e.g., +244 or 244 are both valid).
First digit 9: The number must start with a 9, followed by exactly 8 digits (total length: 9 digits). This change allows phone numbers like 9XXXXXXXX (without the country code) or +2449XXXXXXXX to be matched.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)