Fix sync server version API consistency

[Mirochill]

Summary

• add ConfigClient.get_server_version() to match AsyncConfigClient.get_server_version()
• keep ConfigClient.server_version as a deprecated property that emits DeprecationWarning
• update compatibility checks, tests, docs, and the quickstart example to use the explicit method

Related issues

Closes #52

Test plan

• git diff --check
• git diff --cached --check
• make lint passes - not run locally because this workspace policy forbids running third-party project linters
• make typecheck passes - not run locally because this workspace policy forbids running third-party project type checks
• make test passes - not run locally because this workspace policy forbids running third-party project tests

Checklist

• SDK regenerated if proto changed in decree - not applicable, no proto changes
• Breaking change flagged in title/description if applicable - not applicable, the existing property remains with a deprecation warning
• Changelog/release note added if applicable - not added; this is a small API consistency fix for an open issue

[github-actions]

Thanks for your first pull request! We appreciate the contribution.

Before review, please make sure tests pass: cd sdk && pytest

[Mirochill]

Closing this draft because #52 has now been resolved by the merged maintainer PR #83. Thanks for the quick fix.