Remove pin of outdated R version

[glatterf42]

Today, I was reminded of #479, which recorded some problem of possibly interfering versions 4.2.3 and 4.3.0 of R in our CI test suite. These were encountered in #478, which introduced a workaround by pinning to 4.2.3.

The current release is 4.4.1, so this workaround/pin might be outdated. This PR tries removing the pin to see if our test suite is still working fine with the latest version of R.

How to review

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

PR checklist

• [x] Continuous integration checks all ✅
• [x] Update tests setup
• ~[ ] Add, expand, or update documentation.~ Just CI.
• ~[ ] Update release notes.~ Just CI.

[codecov[bot]]

Codecov Report

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

Project coverage is 98.9%. Comparing base (080ad2b) to head (d3b366c). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #545 +/- ## ======================================= + Coverage 98.8% 98.9% +0.1% ======================================= Files 44 44 Lines 4819 4819 ======================================= + Hits 4765 4770 +5 + Misses 54 49 -5 ``` [see 2 files with indirect coverage changes](https://app.codecov.io/gh/iiasa/ixmp/pull/545/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa)

[glatterf42]

This is working for now, it seems; let's see if it re-introduces some flakiness.