Improve readability of confusion matrix plot

[degiz]

Description of Problem: Originally proposed by @Ghostvv in #3549

Confusion matrix is unreadable (at least for core), color boxes and numbers are not aligned, also numbers overlay:

Overview of the Solution: Come up with proposal on how to improve readability of the plot

Definition of Done:

• Tests are added
• Feature mentioned in the changlog

[sara-tagger]

Thanks for submitting this feature request 🚀@akelad will get back to you about it soon!✨

[erohmensing]

Oops I guess I haven't added you to the tagger yet, it doesn't know you're one of us 😃

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[sync-by-unito]

➤ Maxime Verger commented:

💡 Heads up! We're moving issues to Jira: https://rasa-open-source.atlassian.net/browse/OSS.

From now on, this Jira board is the place where you can browse (without an account) and create issues (you'll need a free Jira account for that). This GitHub issue has already been migrated to Jira and will be closed on January 9th, 2023. Do not forget to subscribe to the corresponding Jira issue!

➡️ More information in the forum: https://forum.rasa.com/t/migration-of-rasa-oss-issues-to-jira/56569.