Upgrade to varnish 7.6

[jmapro]

Is your feature request related to a problem? Please describe.
Varnish 7.3 is no more maintained, an upgrade to varnish 7.5 or 7.6 would be great
https://varnish-cache.org/releases/

Describe the solution you'd like
Update varnish version in the image to the latest one

Describe alternatives you've considered

Additional context

[rebepoi]

I would like that also. Is there any reason why this is not done yet?

[LukeHandle]

I would like that also. Is there any reason why this is not done yet?

Probably waiting for a volunteer/contributor to update the Dockerfile.

If you raise a PR and test the change, this might reduce the burden for the change to be released.

[martin-helmich]

Is there any reason why this is not done yet?

None, other than finding the time for it. PRs and testers are always welcome. 😉