Fix missing Avatar on the schedule user's popup (#4290)

# What this PR does

Fix missing Avatar on the schedule user's popup

## Which issue(s) this PR closes

Closes https://github.com/grafana/oncall/issues/4258

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

grafana-plugin/src/components/ScheduleBorderedAvatar/ScheduleBorderedAvatar.tsx

@@ -79,6 +79,7 @@ const getStyles = () => {
   return {
     root: css`
       position: relative;
+      z-index: 1;
     `,
 
     avatar: css`