ReleaseProcessOverview.md: generalize the Mac/bash-4.x caveat found while exercising Step 12.
package_downloads.sh already had this note narrowly scoped to itself;
gen_file_format_check.sh just hit the identical 'bad substitution' failure
under macOS's stuck-at-3.2 system bash. Moved to a general note in
Preparation (several scripts need bash 4.x+, not just packaging) and
tightened the recommended invocation to an explicit modern-bash path,
since a bare 'bash somescript.sh' can still resolve to the ancient system
one depending on PATH ordering.
Agent: Claude Code, Model: Claude Sonnet 5 (claude-sonnet-5), operating as bot_Claude_Anthropic.