[CI]updated drone.star file to match the latest changes

[SagarGi]

Description

This is an update on drones.star file

• using drone-cancel-previous-builds
• using thegeeklab/drone-github-comment instead of jmccann/drone-github-comment for github comments
• moving docker image references to constants
• using caches images
• Do not run PHP unit tests with Oracle by default
• Skip pipelines based on changed code

Related Issue

• Part of issue:
• https://github.com/owncloud/QA/issues/700
• https://github.com/owncloud/QA/issues/717
• https://github.com/owncloud/QA/issues/729
• https://github.com/owncloud/QA/issues/728
• https://github.com/owncloud/core/issues/38679
• https://github.com/owncloud/QA/issues/730

[CLAassistant]

All committers have signed the CLA.

[phil-davis]

There are still quite a few diffs from the starlark code in the activity app. Please just copy-paste from the activity app.