Update required sundials version in README

[alexander-novo]

Description

Update the required version of SUNDIALS to include specific fixes we need. See this comment by @pelesh.

Proposed changes

List the specific commit needed until a release is made with the fix.

Checklist

Put an x in the boxes that apply. You can also fill these out after creating
the PR. If you're unsure about any of them, don't hesitate to ask. We're here
to help! This is simply a reminder of what we are going to look for before
merging your code.

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.

Further comments

[superwhiskers]

i don't see anything wrong with this.