Newer doxygen

[ischoegl]

Changes proposed in this pull request

• use doxygen from conda rather than ubuntu-latest

Adding conda to the docs runner allows to be more specific with packages not provided by PyPI. Supersedes #1597 (which could not be resurrected as the original branch was deleted).

If applicable, fill in the issue number this pull request is fixing

Closes #1592

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[ischoegl]

Apart from another upstream deprecation that just popped up (this time from Scipy), the newer doxygen fails to generate a list of C++ Modules (missing from left menu). This is likely a new setting ...

[ischoegl]

@speth ... this is ready for a review. I was not able to track down the cause for the 'modules' page no longer being generated for the newest versions of doxygen. For the time being, I pinned doxygen to 1.9.7, as this is the last version not affected by this change (I also updated the Doxyfile, which accounts for most of the diff). In addition, I cherry-picked a fix for the newest CI failures due to deprecations.