gh-36541: `build/pkgs/nauty`: Upgrade to 2.8.6, require nauty >= 2.8
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->
<!-- Why is this change required? What problem does it solve? -->
https://pallini.di.uniroma1.it/changes24-28.txt
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
Fixes #36257.
Continuation of https://github.com/sagemath/sage/pull/36262
We also tighten what versions of system nauty that we accept, because
#36257 was motivated by a new feature. This change should also fix
failures from accepting ancient versions of system nauty, such as
- `ubuntu-xenial-toolchain-gcc_9` https://github.com/mkoeppe/sage/action
s/runs/6622955701/job/18026345225#step:10:7570 in
src/sage/graphs/digraph_generators.py and other files
<!-- If your change requires a documentation PR, please link it
appropriately. -->
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->
- [x] The title is concise, informative, and self-explanatory.
- [ ] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
URL: https://github.com/sagemath/sage/pull/36541
Reported by: Matthias Köppe
Reviewer(s): David Coudert
