Investigate using apt's phased updates post-focal

[legoktm]

In #7333 (comment) @cfm wrote:

It looks like APT has built-in support for phased updates. Could that work for us, so we don't have to implement it ourselves? (This might be worth looking into generally.)

Once we're on noble we can use this functionality. We should investigate if we want to and then how to enable it (reprepro?)

[legoktm]

Note that 29a0f9a is at odds with this. The control for phasing or not is a global setting, so if we want to phase our own updates, we also have to accept phasing of the Ubuntu updates.