Docker: `:latest` tag for `pmm-server` and `pmm-client` images moved from v1 latest release to v2 latest release.

 What should be done:

• We need to start tagging PMM Client/Server docker images with `:latest` tag

Why:

• pmm-server now tagged during pmm1 release process. So we need to move this tag to pmm2 releases
• pmm-client:2 is not tagged with this tag and we need to start this

Benefit:

• we are helping our users who have not checked our Docs and don't see that we recommend they use ":2" and use `:latest` to get the new pmm2 image (instead of pmm1) and be consistent with how it is done in both PMM Client and PMM Server

Details:

Documentation:

• recheck we don't have "latest" in doc 
• mention this is NOT a recommended way