Consider renaming `EnvironmentMapLight` to `EnvironmentLight`

`EnvironmentMapLight` is wordy and post-deprecation of `AmbientLight` in #18207 will be more commonly used. We should prefer simpler, shorter names unless the longer name provides sufficiently important descriptive information.

Start by searching the repository for references to `EnvironmentMapLight` and `AmbientLight`, then read the discussion linked as #18207. Determine whether the shorter name preserves the needed meaning and identify the full set of affected references; done means the naming decision and its consequences are documented clearly enough for implementation.

Written by the indexing model from the issue text.