Bug fix env var names BACKUP_GLOBAL_... for request and limit in helm chart

[Legion2]

Summary

Fixes wrong env var names in helm chart, see this code for references of the correct env var names: https://github.com/k8up-io/k8up/blob/56ca1a38786778474be6260d7c90f65e40a3eff8/cmd/operator/main.go#L65-L68

Checklist

For Helm Chart changes

• [ ] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
• [ ] PR contains the label area:chart
• [ ] PR contains the chart label, e.g. chart:k8up
• [ ] Commits are signed off
• [ ] Variables are documented in the values.yaml using the format required by Helm-Docs.
• [ ] Chart Version bumped if immediate release after merging is planned
• [ ] I have run make chart-docs
• [ ] Link this PR to related code release or other issues.