Restrict dg::Union{DGSEM, FDSBP}

[benegee]

Adapt to trixi-framework/Trixi.jl#2760

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.10%. Comparing base (3cb36b2) to head (f5cbcc2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #142   +/-   ##
=======================================
  Coverage   97.10%   97.10%           
=======================================
  Files          33       33           
  Lines        4558     4558           
=======================================
  Hits         4426     4426           
  Misses        132      132           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.