Fixed regex pattern for dkim_selector #6459

Changed the regex pattern for dkim_selectors again. We now follow the DKIMCore technical specification as stated on the website:

A selector is a string of no more than 63 lower-case alphanumeric characters (a-z or 0-9) followed by a period “.”, followed by another string of no more than 63 lower-case alphanumeric characters.

The discussion that led to the changes: !1692 (comment 106250)

Closes #6459 (closed)