Feat: change aio-stalwart maintainer; BREAKING CHANGE AHEAD in v0.16 - #8556

As discussed with @docjyJ in docjyJ/aio-stalwart#123, I'll take over the maintenance of the aio-stalwart community container.
Where I want to bring out a big THANK YOU to @docjyJ to set up this container and all the efforts you put into it.

This PR changes the container image from docjyJ to winterrific and the references in the documentation. Currently, there are no improvements done in the container itself. However, the next two releases are planned, where the
1st introduces an export script to save and convert the v0.15 settings, which then will get imported in the
2nd including the upgrade to v0.16 of stalwart.

v0.16 has a BREAKING change in how stalwart is configured and managed. So far, Stalwart configuration is stored in a TOML file. In v0.16 all settings are stored in the database and the TOML file is gone. Thus, the settings have to be exported first and then imported in the to v0.16 upgraded container image.

Summary

• The PR was tested and verified that it works locally
• Sign-off message is added to all commits
• Tests (playwright if possible) are included
• Screenshots before/after for front-end changes
• Documentation has been updated or is not required
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone next added

AI (if applicable)

• The content of this PR was partly or fully generated using AI