Fix animated "Loading..." message keyframes

[cmdcolin]

Was lost during the mui4->mui5 transition

Fixes #3307

[codecov]

Codecov Report

Merging #3309 (7bc7ee5) into main (14aa9fa) will increase coverage by 0.00%.
The diff coverage is 78.94%.

❗ Current head 7bc7ee5 differs from pull request most recent head 72c9160. Consider uploading reports for the commit 72c9160 to get more accurate results

@@           Coverage Diff           @@
##             main    #3309   +/-   ##
=======================================
  Coverage   59.44%   59.45%           
=======================================
  Files         735      736    +1     
  Lines       28985    28989    +4     
  Branches     7012     7013    +1     
=======================================
+ Hits        17230    17235    +5     
+ Misses      11509    11508    -1     
  Partials      246      246           

Impacted Files	Coverage Δ
.../core/BaseFeatureWidget/SequenceFeatureDetails.tsx	38.93% <0.00%> (ø)
packages/core/ui/App.tsx	76.71% <ø> (ø)
packages/core/ui/DrawerWidget.tsx	80.00% <ø> (ø)
...onnectionWidget/components/ConfigureConnection.tsx	100.00% <ø> (ø)
...comparative-view/src/ServerSideRenderedContent.tsx	0.00% <0.00%> (ø)
...play/components/ServerSideRenderedBlockContent.tsx	100.00% <ø> (ø)
...browse-desktop/src/StartScreen/QuickstartPanel.tsx	0.00% <0.00%> (ø)
products/jbrowse-web/src/Loader.tsx	61.90% <50.00%> (ø)
packages/core/ui/AboutDialog.tsx	75.80% <100.00%> (ø)
packages/core/ui/LoadingEllipses.tsx	100.00% <100.00%> (ø)
... and 7 more

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