[MA-32]: Added logic to correctly give accessible name to channel header button

[ayush-chauhan233]

Summary

• Added aria-labelled by to system bot heading in the channel header
• Updated the aria-label on the channel header button to announce the channel name correctly.
• Removed aria-label from the disable calls button

Steps to reproduce

• Locate the Town Square button control.
• Inspect it with Chrome DevTools.
• In the Accessibility tab, expand the Computed Properties section.
• Review the value for "Name".
• Notice that this value does not include the visible text of the control as a substring.
• Review the value for "Description".

Expected Behavior

• The screen reader must announce the accessible button name correctly.

Ticket Link

Jira https://mattermost.atlassian.net/browse/MM-61646

Screenshots

Release Note

NONE