Use Docker daemon/CLI 20.10.6 instead of 19.03.0 on CI

What does this Merge Request do and why?

Uses Docker daemon/CLI 20.10.6 instead of 19.03.0 on our CI to improve security and performance. Removed option DOCKER_CLI_EXPERIMENTAL is also removed from the CI configuration.

https://github.com/docker/cli/blob/a32cd16160f1b41c1c4ae7bee4dac929d1484e59/docs/deprecated.md#configuration-options-for-experimental-cli-features

Configuration options for experimental CLI features

The DOCKER_CLI_EXPERIMENTAL environment variable and the corresponding experimental field in the CLI configuration file are deprecated. Experimental features will be enabled by default, and these configuration options will no longer be functional.

code_quality, container-scanning, release-image, build-gitpod-workspace-image, and deploy-gitpod-workspace-image jobs are relevant.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1228 (closed)