Fix stale runtime version string and automate version bumps

[whiteh]

Fix stale runtime version string and automate version bumps

Summary

• _N._VERSION in src/lua_resty_netacea.lua was still hardcoded to 1.6.0 despite the rockspec having moved to 1.6.2-0 in an earlier release — so require("lua_resty_netacea")._VERSION reported the wrong version at runtime. Bumped it to 1.6.3 in lockstep with a new rockspec (lua_resty_netacea-1.6.3-0.rockspec).
• Extended check_rockspec_version.sh to also assert that _N._VERSION matches the rockspec's release version, so this can't silently drift again — it previously only checked the rockspec filename against its own version field and against existing git tags.
• Added bump_version.sh <new-version> to automate the release checklist: renames the rockspec, updates its version field, updates _N._VERSION, and runs check_rockspec_version.sh to confirm everything agrees.
• Updated CONTRIBUTING.md's "Updating the version number" section to point at the new script instead of the old manual 3-step checklist.

Why

Manually keeping the rockspec version and the in-code version string in sync is easy to forget, and check_rockspec_version.sh had no way to catch it — it verified the rockspec was internally consistent but never looked at the module source. This fixes today's instance of the bug and closes the gap so CI catches the next one.

Test plan

• ./check_rockspec_version.sh passes against the new rockspec/version.
• ./bump_version.sh 1.6.4 exercised manually (renamed rockspec, updated both version strings, ran the check successfully), then reverted back to 1.6.3 since that was just a dry run.
• CI (build.yml) runs check_rockspec_version.sh as before — should still pass on this branch.