purevpn: update version, livecheck

[AlternateRT]

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In the following questions <cask> is the token of the cask you're submitting.

After making any changes to a cask, existing or new, verify:

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This PR removes the livecheck block and changes the version to :latest because the page used for livechecking is often outdated for months with no end - the page was last updated over two months ago, and since that time, two newer versions have come out.

I left the block commented to remember the URL, just in case the page is properly updated in the future.

[bevanjkay]

Rather than removing the livecheck, we could try to use the extract_plist livecheck strategy on the url, if it returns the correct version it is likely the best option here.

[AlternateRT]

Sorry for disappearing almost immediately after creating this PR 😅

Just for future reference, using :extract_plist livecheck is still preferred over using version :latest? When I opened this, I refrained from changing the livecheck back to :extract_plist because of #171006 and #180264.

My thinking was that since :extract_plist should only be used as a very last resort due to how costly it is, and this cask was already moved away from using, then using version :latest would have been preferable; the download URL is already unversioned after all, and always provides the latest version.