Change username validation message

What does this MR do and why?

Validation message has been changed from:

• Username must not start or end with a special character and must not contain consecutive special characters.

to:

• Username can only include letters, digits, '_', '-' and '.'. It must not start with '-', end in '.', '.git', or '.atom'. Avoid using accented characters or symbols.

These changes will affect the following areas that uses oci_repository_path_regex_message:

• Namespace model
• Project model
• Bulk imports validator
• Bulk imports error

Related to #429283 (closed)