891: Update fetchmail to selfbuilt 7.0.0-alpha r=mergify[bot] a=Nebukadneza
## What type of PR?
enhancement
## What does this PR do?
Fetchmail in alpine is ~5 years old — and doesn’t support current SSL/TLS
variants anymore. This especially leads to our own fetchmail not being able to
pull mail from mailu itself. Since no new fetchmail release is on the horizon,
let’s build the lastest distribution artifact — which strangely is not
6.4.0-snapshot, but 7.0.0-alpha — ourselves.
### Related issue(s)
closes#808
## Prerequistes
- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: place entry in the [changelog](CHANGELOG.md), under the latest un-released version.
Co-authored-by: Dario Ernst <dario@kanojo.de>
Co-authored-by: Nebukadneza <github@kanojo.de>
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