deepl: update livecheck and legacy versions

[brzzdev]

The existing implementation of DeepL used regex to search for the 'most recent' version, which could often result in finding incorrect pre-release versions. (See mangerlahn/Latest#447)

I've simplified this down to just using https://www.deepl.com/macos/download/DeepL.dmg which will always point to the latest release version, and I've removed the no longer supported legacy versions.

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[khipp]

It makes sense to update the livecheck to use the Sparkle feed. However, we should continue using the versioned URL, as it allows for the computation of checksums.

Instead of removing the legacy versions, we could use skip "Legacy version" in the livecheck block for macOS Catalina and older, as they no longer receive updates.

[brzzdev]

Keeping the livecheck makes sense but I'm not sure I agree on keeping all the legacy versions.

They used to be listed on deepl.com, but they've now been removed and no support for them is offered.

[khipp]

Keeping the livecheck makes sense but I'm not sure I agree on keeping all the legacy versions.

I don't see any downside to keeping the legacy versions unless they are known to have issues or make the cask harder to maintain. @bevanjkay, I would love your thoughts on this.

[reitermarkus]

As long as the download URLs for the legacy versions work, I also don't see a reason for removing them.