Suggestion: Update sphinx pinnings

[AlenkaF]

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [ ] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [ ] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [ ] Ensured the license file is being packaged.

As Breathe itself is now testing with Sphinx 6 (https://github.com/breathe-doc/breathe/commit/ef4e245804759d44ae54fc832b4d5155837f0720) we would like to suggest pinning the Sphinx version to <7. This would enable the build of the documentation with Sphinx 6 and newer version of Breathe.

[conda-forge-webservices[bot]]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[hmaarrfk]

the bulid number also needs to be increased

[hmaarrfk]

Great. Thank you!