Reduce goroutine count for processing expiration func

[kpango]

This pull request introduces improvements to the concurrency handling for expired item processing in the gache cache implementation. The main changes are the adoption of the errgroup package for managing goroutines, which allows for better control and error handling, and the addition of the new dependency in go.mod.

Concurrency improvements:

• Replaced manual goroutine spawning in StartExpired with errgroup, enabling structured concurrency and limiting the number of concurrent goroutines to twice the number of CPU cores. This ensures more robust error handling and resource management.
• Changed context handling in StartExpired to use the errgroup context, improving cancellation and synchronization of goroutines.

Dependency updates:

• Added golang.org/x/sync (which provides errgroup) to go.mod to support the new concurrency model.
• Imported golang.org/x/sync/errgroup in gache.go to enable usage of the errgroup package.