ci: downgrade container to ubuntu 20.04

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR downgrades container version to Ubuntu 20.04

Checklist

• I have written the necessary comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

[MichaelScofield]

I understand the necessity of downgrade the building job of protobuf to ubuntu 20, it's related to some lib dependencies versions. But why other jobs?

[evenyag]

I understand the necessity of downgrade the building job of protobuf to ubuntu 20, it's related to some lib dependencies versions. But why other jobs?

Just keep all jobs using the same OS version. Some jobs use the latest version, and some jobs use 20.04 is confusing.