[8/n] [reconfigurator] add target release generation to blueprint

[sunshowers]

As described in RFD 556, we need this field as a backstop against Nexus overriding a freshly-conducted mupdate.

There is small semantic change here compared to RFD 556, though not one of much consequence:

• As currently described in RFD 556, Nexus believes it is in control if the target release generation is greater than this field.
• In this PR, that logic has been changed to greater than or equal to.

This has two advantages:

1. It aligns somewhat better with the meaning of "minimum".
2. target_release_minimum_generation no longer needs to be an Option<Generation>; it can be a Generation instead. That's because we always insert a generation 1 in the target_release table as "unspecified", meaning to use the install dataset. So setting this field to generation 1 just means "use the install dataset".

[jgallagher]

The wording on this is correct but I read it incorrectly the first time through, because "the target release generation" is pretty similar to the name of this field; I was confused about how the target release generation might be set to 6 in the future when we've already set this field to 6. The event we're waiting for here is "an operator sets a new target release" (which will necessarily bump the generation number up to 6), right? And we also have to have cleared all the mupdate overrides in the meantime (or as a part of that new release being set)?

[sunshowers]

I've updated it to have a more active voice:

    /// Once an operator sets a new target release, its generation will be 6 or
    /// higher. Reconfigurator will then know that it is back in charge of
    /// driving the system to the target release.

And we also have to have cleared all the mupdate overrides in the meantime (or as a part of that new release being set)?

I believe that's right, yes.