Remove code and CI for UE 5.4

[j9liu]

Description

Now that our new minimum version is UE 5.5, this removes all #if directives that preserved compatibility with UE 5.4.

Builds off of #1770.

Author checklist

• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

Although a holistic samples spot-check would be great, make sure that in particular:

• Point cloud shading still works
• Material generation maintains the same level of functionality

[j9liu]

The following docs need non-trivial edits, so I haven't touched them yet:

• developer-setup-linux.md
• developer-setup-windows-gotchas.md (are these all applicable still with 5.5?)

developer-setup-osx.md could also use a look from a dedicated Mac user :')

[j9liu]

• @kring - from a glance, do you know if the Windows gotchas still apply to 5.5? (is it just a matter of changing version numbers, or something else?)
• @timoore - can you check out what needs to change for 5.5 in developer-setup-linux.md?
• @david-lively - can you verify that my changes to developer-setup-osx.md are correct for Unreal 5.5?