chore: remove aws-sdk-go-cache dependency

[tekenstam]

Summary

Remove the `keikoproj/aws-sdk-go-cache` dependency from lifecycle-manager. The library is built on AWS SDK for Go v1 (EOL July 2025) and provides response caching that is no longer needed.

Background

See keikoproj/instance-manager#525 for the full deprecation rationale.

TL;DR: AWS API rate limiting hasn't been an issue in practice, and AWS SDK v2's built-in standard retry mode (token-bucket rate limiting + exponential backoff) handles throttling well out of the box.

Current Usage

lifecycle-manager caches 6 ELB/ELBv2 operations:

Service	Operation	TTL
ELBv2	DescribeTargetHealth	120s
ELBv2	DescribeTargetGroups	300s
ELBv2	DeregisterTargets	(non-mutating flag only)
ELB	DescribeLoadBalancers	300s
ELB	DescribeInstanceHealth	120s
ELB	DeregisterInstancesFromLoadBalancer	(non-mutating flag only)

Files to Modify

• `cmd/common.go` — remove cache config and `NewCacheConfig`
• `cmd/serve.go` — remove cache import
• `go.mod` / `go.sum` — remove `keikoproj/aws-sdk-go-cache` dependency

Migration Path

This can be done standalone or as part of a future AWS SDK v2 migration:

• Replace `cache.NewSession()` with standard session/config construction
• The `DeregisterTargets` / `DeregisterInstancesFromLoadBalancer` non-mutating flags become irrelevant (they only prevented cache invalidation)
• AWS SDK v2 standard retry mode is the default — no extra code needed
• Run `go mod tidy` to clean up