Log filters: Replace "form group header" with "dropdown header"

What does this MR do?

To make better use of space the form group header is removed and dropdown header is enabled in two log filter dropdowns.

• Height of filter box is adjusted to match the dropdown appearance
• "selected" (checked) icons are added to match the design spec
• Order of dropdowns is changed, in relation to #207912 (closed)

Product + technical iteration of #207912 (closed)

Screenshots

Before

Environment Dropdown	Pods Dropdown

After

Environment Dropdown	Pods Dropdown

Pods dropdown loading state:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team