Update minimum version of genno

[glatterf42]

As noted by @awais307, we should bump the minimum required version of genno to the one that introduces genno.operator.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅
• ~[ ] Add or expand tests; coverage checks both ✅~ Just bumping dependency
• ~[ ] Add, expand, or update documentation.~ Just bumping dependency
• ~[ ] Update release notes.~ Just bumping dependency

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (49ec67d) 98.9% compared to head (aa75739) 98.2%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #514 +/- ## ======================================= - Coverage 98.9% 98.2% -0.8% ======================================= Files 44 44 Lines 4795 4795 ======================================= - Hits 4744 4709 -35 - Misses 51 86 +35 ``` [see 3 files with indirect coverage changes](https://app.codecov.io/gh/iiasa/ixmp/pull/514/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa)

[khaeru]

This should also go in the release notes. For examples, see: https://github.com/iiasa/ixmp/blob/49ec67dc41dd91bee52879dbfbd83f24064d5f76/RELEASE_NOTES.rst?plain=1#L75

https://github.com/iiasa/ixmp/blob/49ec67dc41dd91bee52879dbfbd83f24064d5f76/RELEASE_NOTES.rst?plain=1#L91-L94

[glatterf42]

Merging this since the coverage is not affected, codecov just didn't register the reports correctly.