feat: change default auth method to md5

Summarize the changes

• feat: change default auth method to md5

The change is required according to the ticket picodata#876 (closed)

Ensure that

- [ ] New code is covered by unit and integration tests.

- [ ] Related issues would be automatically closed with gitlab's closing pattern (Closes #1, #2).

- [ ] Public modules are documented (check the rendered version with cargo doc --open).

- [ ] (if PEST grammar is changed) EBNF grammar reflects these changes (check the result with railroad diagram generator.

Next steps

• Cherry-pick to: none
• Update sbroad submodule in picodata/picodata.
• (if EBNF grammar is changed) create a follow-up issue in picodata/docs.