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.

Enable DKIM when generating a Private-key #6538

See merge request ispconfig/ispconfig3!1766

Merge branch '6533-domain-module-prevents-subdomains-to-be-added-for-domains-that-exist-as-website' into 'develop'

Resolve "Domain module prevents subdomains to be added for domains that exist as website"

Closes #6533

See merge request ispconfig/ispconfig3!1764

Resolve "DNS Zone list server column empty"

Closes #6524

See merge request ispconfig/ispconfig3!1760

Resolve "API dkim_selector_error in mail_domain_add ()"

Closes #6523

See merge request ispconfig/ispconfig3!1759

Resolve "DNS Wizard not working with use_domain_module = y"

Closes #6519

See merge request ispconfig/ispconfig3!1758

Merge branch '6522-dns-zones-can-not-be-added-with-wizard-when-domain-module-is-enabled' into 'develop'

Resolve "DNS zones can not be added with Wizard when domain module is enabled."

Closes #6522

See merge request ispconfig/ispconfig3!1757

Replace one more occurance of slave_zone_file_prefix, #6520

See merge request ispconfig/ispconfig3!1755

Fix mixed up version numbers in check_mysql_version

See merge request ispconfig/ispconfig3!1750

Improve verbosity when an error occurs during replication

Closes #6510

See merge request ispconfig/ispconfig3!1747

Only display a dashlet table if there is content for it, #6512

See merge request ispconfig/ispconfig3!1749

6516_help_system_message_sort_fix

See merge request ispconfig/ispconfig3!1752

Resolve "Error sending support message in ISPConfig panel"

Closes #6515

See merge request ispconfig/ispconfig3!1751

Integrate handle_mailbox_soft_deleted.sh into 500-clean_mailboxes.inc.php

Closes #6445

See merge request ispconfig/ispconfig3!1733

Resolve "Show DKIM key in unsplit format"

Closes #6454

See merge request ispconfig/ispconfig3!1711