Update nektar-env to bullseye

Issue/feature addressed

This merge request moves the Docker images over to the latest Debian stable (bullseye) and fixes a small issue with the Python wrapper that prevented it from finding the installed Nektar++ libraries (default location is /usr/local/lib64 and this is now adjusted to /usr/local/lib which is a default system path).

Proposed solution

See above.

Implementation

See above.

Tests

Small test added to

Notes

N/A

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Newly added files are correctly formatted.
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).