Log Message:
balsa: update to 2.6.3

This fixes the STARTTLS-related crash bugs mentioned here:
https://nostarttls.secvuln.info/

* Balsa-2.6.3 release. Release date 2021-08-18

- Improve Autocrypt-related error messages.
- Improvements to communication with GnuPG key servers.
- Create standard-compatible HTML messages.
- Implement sender-dependent HTML message preferences.
- Reuse HTTP connections when rendering HTML messages.
- Do not send empty Reply-To, Cc, etc headers.
- More robust IMAP parser and response handling.
- Code cleanups, platform-dependent build fixes