Update to 6.1.2

Changelog:

• Substantial revision of DKIM header generation
• Use shorter hashes for auto-generated CID values
• Fix format of content-id headers, and only use them for inline attachments
• Remove all use of XHTML
• Lots of coding standards cleanup
• API docs are now auto-updated via GitHub actions
• Fix header separation bug created in 6.1.1
• Fix misidentification of background attributes in SVG images in msgHTML

Sponsored by: Netzkommune GmbH