Make logic for testing the operator upgrade paths extensible

[rm3l]

/kind user-story
/area testing

User Story

As an operator developer, I want to easily extend the current operator upgrade E2E tests to include additional supported versions, So that I can be confident the operator can be upgraded from one supported version to another.

This is a follow-up issue related to this review comment: #384 (comment)

Acceptance Criteria

• It should extend the current approach, which relies on a specific deployment manifest (0.1.3) as the starting point - see https://github.com/janus-idp/operator/blob/main/tests/e2e/e2e_upgrade_test.go#L53-L54
• It should allow to try out different upgrade paths, using different starting versions to the current one.
• It should be documented in the E2E tests README
• ...

Links

• Related to the Operator Level 2 Capability Level: https://sdk.operatorframework.io/docs/overview/operator-capabilities/#level-2---seamless-upgrades
• JIRA: https://issues.redhat.com/browse/RHIDP-2641

/kind user-story

[rm3l]

JIRA issue: https://issues.redhat.com/browse/RHIDP-2641