bump scheduledworkflow rock to ckf-1.8 (kfp 2.0.3)

[orfeas-k]

• bump source version from 2.0.0-alpha.7 to 2.0.3
• update according to Standardize the ROCKs tags we use bundle-kubeflow#747
• rename base to ubuntu@20.04 as per new rockcraft syntax
• use 1.20/stable snap for Go
• refactor tox.ini according to add tests to OIDC authservice rock oidc-authservice-rock#14 and Refactor scripting part from tox.ini files in ROCKs repos bundle-kubeflow#763
• use placeholder instead of bundle-integration tests due to bundle-integration tests fail to build charm in the CI  #61 (there are no charm integration tests for this charm)
• add placeholder instead of sanity tests since there are none at the moment

Note CI needs a sanity and integration environment in order to not fail and be able to publish the produced ROCK after merging this.

Testing

ROCK can be built and unit goes to active using it. I didn't run any hands-on tests since bundle-integration and manual tests will be run once all kfp ROCKs are updated.

Closes #46