RTD: Update OS version to ubuntu-lts-latest

[MikeOpenHWGroup]

Automatic re-generation of CVA6 documentation on ReadTheDocs is failing because of Ubuntu-20.04 is no longer supported.

[ASintzoff]

should we not use a specific version like ubuntu-26.04? Doing so will avoid any possible issue when ubuntu-lts-latestwill be switching to ubuntu-28.04.

This reminds me of #3443 by @cainria where Verible version is now explicitly specified

[cainria]

I agree with @ASintzoff but my opinion on the OS version freeze is weaker than on the Verible version freeze

The issue with Verible was that changing the formatting impacts all files, so all contributors have to upgrade simultaneously. For the Read the Docs OS, I think the issues would be mainly in the list of packages to install in the runner, which do not impact contributors.

Apart from that, freezing is better for reproducibility, but ubuntu-26.04 will eventually be deprecated in the long-term so we will need to change the version explicitly (preferably before the jobs fail next time 😉)

[cainria]

Following @ASintzoff suggestion:

Suggested change

	os: "ubuntu-lts-latest"
	os: "ubuntu-26.04"

And for reference, here are the docs: https://docs.readthedocs.com/platform/stable/config-file/v2.html#build-os