Allow sample rate, number of channels and bps to change and in that case update
the caps accordingly.
Also move (non-fatal) validity checks and storing of the header values outside
the actual parsing once we actually know that a valid frame is available.
And also don't warn on the last frame with fixed block size blocking strategy
that the block size has changed: the last frame is allowed to be smaller.
Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3281
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8075>