Handle encoding issues in loading credits.html

[llimeht]

Description

Be defensive against non-UTF-8 chars that can creep into credits.html since it is compiled from lots of different sources, and force use of UTF-8 since the file should already be UTF-8 via pip-licenses and build_tools/compile_licenses.py.

This PR

• forces use of UTF-8 for credits.html since that is what it should be, working around the on-going obsession windows has with cp1252 (as spotted in the traceback in Credits and Licences throws error #4013) ... bring on Python 3.15 for that to finally change.
• makes reading credits.html robust against malformed data by using replacement characters where it needs to, in order to ensure it can decode from UTF-8.
• switches loading credits.html to the established code path within system.legal
• fixes a latent bug in the credits_html function (function is not used elsewhere in the code bae)

Fixes #4013

How Has This Been Tested?

Tested locally on linux (but note that this is a windows-specific bug due to it still using cp1252):

. .venv/bin/activate
python build_tools/compile_licenses.py src/sas/system/credits.html
sasview

Have not tested from the installer because generating installer images has been disabled in CI.

Note that unrelated failing test on linux is #4000 which is waiting for PRs to be merged to fix.

Review Checklist:

[if using the editor, use [x] in place of [ ] to check a box]

Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)

[krzywon]

Code looks good. Seems like this should help to fix the issue, but I haven't tested.