Upgrade to Claws-Mail 3.21

[cobratbq]

Upgraded to Claws-Mail 3.21 (Upgrading to Claws-Mail 4.20 or 4.21 results in plugins being impossible to load, #34, because the file-loading dialog no longer provides access to flatpak application content.)

Includes:

• 38

• 39

• 37

• 36

• 35, GData is no longer supported, so the plugin will disappear.

[flathubbot]

Started test build 129270

[flathubbot]

Build 129270 failed

[flathubbot]

Started test build 129295

[flathubbot]

Build 129295 failed

[flathubbot]

Started test build 129314

[flathubbot]

Build 129314 failed