common: migrate the deprecated --mmap/--no-mmap to --load-mode (#26934)
Replace the deprecated --mmap, --no-mmap, --mlock, and --direct-io flags with
the unified --load-mode argument across scripts, examples, and documentation.
Internal warning message and env var docs updated accordingly.
Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>