fix: update cursor to pointer on hover for username (#8317)

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### Motivation and context
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As cursor don't turn into pointer so it is very misleading to the end
user. They might confuse whether this is click able button or not. (fix
#8316)

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Run all test cases also check related UI manually to insure double
check.

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Enhanced user interface by indicating interactivity of the user
dropdown menu with a new pointer cursor style.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Kirill Lakhov <kirill.9992@gmail.com>

cvat-ui/src/components/header/styles.scss

@@ -60,6 +60,7 @@
 .cvat-header-menu-user-dropdown {
     display: flex;
     align-items: center;
+    cursor: pointer;
 
     .anticon.cvat-header-dropdown-icon {
         &.anticon-caret-down {