Move boards-sidebar class into content_sidebar

What does this MR do?

This MR fixes a visual regression introduced by !62051 (comment 609952997) since it changes the root component of a couple of components preventing class="..." from getting passed through.

Screenshots (strongly suggested)

Description	Before	After
With Feature.enable(:graphql_board_lists) on issue board
With epic board

Does this MR meet the acceptance criteria?

Conformity

• [ ] I have included changelog trailers, or none are needed. (Does this MR need a changelog?) Not needed since fixed in same milestone introduced.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.