[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".