Frontend: Update Duo metrics table to display MAU-based metrics

Update the Duo metrics table in the analytics dashboard to display MAU (Monthly Active Users) based absolute numbers for classic/legacy Duo features.

Tasks

• Convert "Duo features usage (Classic)" to display as number of contributors who used any classic GitLab Duo feature (MAU regardless of seat assignment)
• Convert "Code Suggestions usage (Classic)" to display as number of MAU using Code Suggestions
• Convert "Duo Chat usage (Classic)" to display as number of MAU who used legacy Duo Chat
• Convert "Duo RCA usage (Classic)" to display as number of MAU who used RCA
• Ensure all metrics no longer consider historical seat assignment, but rather MAU

Update metric labels and descriptions

Duo features usage

• Update description to "Number of users who used any GitLab Duo feature."

Code Suggestions usage

• Update label to "Code Suggestions usage (Classic)"
• Update description to "Number of users who used Code Suggestions."

Duo Chat usage

• Update label to "Duo Chat usage (Classic)"
• Update description to "Number of users who used Duo Chat."

Duo RCA usage

• Update label to "Duo RCA usage (Classic)"
• Update description to "Number of users who used RCA."

Related to

Epic #20540 (closed) - Section 7: Duo metrics table improvements