setup-oras is not required for Ubuntu runners

[shizhMSFT]

We should mention in README.md that the latest oras CLI is pre-installed in GitHub-hosted runners used for Actions, as well as for Microsoft-hosted agents. Therefore, setup-oras is not required for those runners to use the oras CLI.

[CodeMan99]

Important to link the documentation for the runner image.

https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2204-Readme.md#cli-tools

When implementing, I want to know that the tool I'm using is intentionally provided.

[BethGriggs]

Just to check, will this still be the case for Ubuntu 24.04 runners?

From actions/runner-images#9848:

ORAS CLI latest available - Removed from the Ubuntu 24.04 image due to maintenance reasons.

[shizhMSFT]

@BethGriggs The ubuntu 24.04 is currently a beta image and is not finalized yet.

Here are some comments I found in the issue actions/runner-images#9848:

Q: actions/runner-images#9848 (comment)

Are the changes marked "Removed from the Ubuntu 24.04 image due to maintenance reasons." final? Is there a particular common reason? Can we somehow vote/plead to revert any of those choices?

I expect a lot of builds with "ubuntu-latest" to break if it hits GA in that form.

A: actions/runner-images#9848 (comment)

there will definitely be changes until GA including the software list installation (for instance we are to add firefox & edge, but a bit later), but we do not guarantee we are to address all the requests, everybody is welcome to file new tools addition requests and we'll review them.