Refactor remaining SDL2 references to remove version specificity and improve clarity

[FelixWolf]

Description

This removes "SDL2" from the remaining variable names in order to improve clarity of the function. This has no functionality change, this is code clean up only.

I went with just "SDL" over naming them "SDL3", as "SDL" refers to just Simple DirectMedia Layer, rather than a specific version. Should SDL4 come out and the viewer upgrade to it, this makes less stuff to rename in the future. This also aligns with various other variable names.

Related Issues

• Please link to a relevant GitHub issue for additional context.
  • Bug Fix: Link to an issue that includes reproduction steps and testing guidance.
  • Feature/Enhancement: Link to an issue with a write-up, rationale, and requirements.

Issue Link: N/A

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Additional Notes

This DOES NOT change media_plugins/cef/media_plugin_cef.cpp: mCEFLib->nativeKeyboardEventSDL2(key_event, native_virtual_key, native_modifiers, keypad);, which is the last remaining entry for this. This requires modifying the CEF lib which I am uncomfortable with doing.

[FelixWolf]

I accidentally had the base branch to merge into set to develop instead of develop-linux, as such github-actions added all the tags it possibly could, even after I changed the base branch to develop-linux.
90% of them can be removed, this only affects c/cpp and llwindow.

[Geenz]

LGTM