[MINOR][BUILD] Mark spark-deps manifest as auto-generated

[pan3793]

What changes were proposed in this pull request?

dev/test-dependencies.sh now writes a header at the top of each manifest it generates (dev/deps/spark-deps-*):

#
# Generated by `dev/test-dependencies.sh --replace-manifest`.
# Do not edit manually.
#

The same header is prepended to the committed dev/deps/spark-deps-hadoop-3-hive-2.3 so the manifest matches generator output and the self-check (dev/test-dependencies.sh without --replace-manifest) stays green.

Only POSIX-portable utilities are used in the new code path (printf '%s\n', cat, rm); the existing awk already uses only POSIX-standard functions, so the script remains compatible across macOS (BSD awk) and Linux (gawk).

Why are the changes needed?

The manifest is machine-generated and regenerated by dev/test-dependencies.sh --replace-manifest; hand edits get overwritten on the next regeneration. A header stating this prevents accidental manual edits to a checked-in generated file.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

• bash -n dev/test-dependencies.sh
• Verified byte-for-byte that the header the script emits matches the header prepended to the committed manifest, so the manifest self-check still passes.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: GLM 5.2

[pan3793]

I find some agents tend to modify it manually without this guidance, for example, #57779

[uros-b]

LGTM, thank you @pan3793