Consolidate temporal layer limit to kMaxTemporalStreams

The VP8 and VP9 RTP reference finders defined their own local constant,
`kMaxTemporalLayers`, with a value of 5. This was inconsistent with the
global constant `kMaxTemporalStreams`, which has a value of 4 and is
used for this purpose elsewhere.

This patch removes the redundant local `kMaxTemporalLayers` definitions
in `rtp_vp8_ref_finder.h` and `rtp_vp9_ref_finder.h`. All usages (array
bounds, frame validation checks, and log messages) are updated to use
the global `kMaxTemporalStreams` constant instead.

This change aligns the validation logic with the rest of the codebase,
dropping frames with a temporal index of 4 or greater.

Fixed: chromium:456855150
Change-Id: I0de0b2b765ccf9bef30dfbca504b9c9f7179dabe
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/422720
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48002}