[Part 1] Clean up :apollo_boards feature flag

What does this MR do and why?

Clean up :apollo_boards feature flag

Remove mentions of :apollo feature flag for the following components

• BoardSiderBarTitle
• BoardApp
• BoardCardInner
• BoardCardMoveToPosition
• BoardColumn
• BoardContent
• BoardTopBar
• BoardNewIssue
• BoardForm
• BoardListHeader
• BoardFilteredSearch

Progress of the feature flag removal

Steps	MR
Part 1	You are here
Part 2	!138006 (merged)
Swimlanes	TBD
Delete VueX store	TBD

Screenshots or screen recordings

No visual changes

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #430042 (closed)