Stop writing v1 ActiveSession keys

What does this MR do and why?

This MR removes the setex command for v1 session keys. With this, the application will maintain read-support for v2 and v1 keys but only write v2 keys. This removal can be performed now without breaking self-managed users since fallback lookups are performed to read any existing v1 keys.

Removing the setex commands give us immediate benefits in reducing CPU load on redis-sessions and resolves gitlab-com/gl-infra/scalability#2215 (closed)

For context, the mixed deployment mode with v2 keys was introduced in v14.6.

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