[CI] Updated drone.star file to match latest changes

[grgprarup]

Description

This is an update on drone.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:

• 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.