fix(clustering/rpc): inert rpc protocol by default in 3.9

[chronolaw]

Summary

KAG-5908

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #[issue number]

[AndyZhang0707]

should we set it to inc_sync?

[chronolaw]

Not sure about it, but I think that cp should always support two prorocols.

[chobits]

i know andy's meaning, but it need some effort to enable it, he want us to test the following 3 kinds of scenarios:

1. rpc = off, inc_sync = off <-- and this one is our default action, this is not tested in these test cases
2. rpc = on, inc_sync = off
3. rpc = on, inc_sync = on

the double for-loop might be tricky:

for rpc in {off, on}
   for inc_sync in {off, on}
      if rpc == off, inc_sync == on then
          continue
      end
      ... run test ...

or

--- gpt generated code
for _, pair in ipairs({{"off", "off"}, {"on", "off"}, {"on", "on"}}) do
   print("rpc: " .. pair[1] .. ", inc_sync: " .. pair[2]) end

 ...

[chronolaw]

done, thank you.