chore: deprecate and remove aws-sdk-go-cache dependency

[tekenstam]

Summary

keikoproj/aws-sdk-go-cache should be deprecated and removed from all consumer repos. This issue tracks the deprecation rationale and cross-repo coordination. The actual removal in instance-manager is part of the AWS SDK v2 migration (#523).

Background

aws-sdk-go-cache is a response-caching layer for AWS SDK for Go v1 that caches read-only API calls (Describe*, List*, Get*) in-memory. It was originally built to mitigate AWS API rate limiting (throttling).

Why Remove It

1. AWS SDK v1 is EOL — the library is built entirely on v1 request handlers with no v2 equivalent
2. Rate limiting is no longer a practical issue — throttling hasn't been observed in keikoproj deployments in a long time
3. AWS SDK v2 has robust built-in retry — standard retry mode includes token-bucket rate limiting (500 tokens), exponential backoff with jitter, configurable max attempts; adaptive retry mode adds dynamic client-side rate adjustment
4. Maintenance burden — porting to v2 SDK middleware would be a significant rewrite for a problem that doesn't exist
5. Short TTLs provide marginal benefit — most cached operations use 60s TTLs; built-in retry handles transient throttling equally well
6. Complexity cost — TTL tuning, invalidation bugs, stale data risks

Consumer Repos

Repo	Cached Operations	Most Aggressive TTL	Tracking Issue
instance-manager	16 ops (EC2, ASG, IAM, EKS, SSM)	24h (`DescribeInstanceTypes`)	This issue + #523
upgrade-manager	2 ops (ASG, EC2)	60s	keikoproj/upgrade-manager#544
lifecycle-manager	6 ops (ELB, ELBv2)	300s	keikoproj/lifecycle-manager#271

Migration Path

Remove the dependency as part of the AWS SDK v2 migration (#523):

• Replace `cache.NewSession()` / `cache.NewCacheConfig()` with standard `config.LoadDefaultConfig(ctx)`
• AWS SDK v2 standard retry mode is the default — no extra code needed
• For `DescribeInstanceTypes` (24h cache — truly stable data), add a simple `sync.Once` (~10 lines) instead of maintaining an entire caching library
• Remove Prometheus cache metrics (`aws_api_cache_activity`, `aws_api_cache_flushes`); replace with v2 middleware if observability of API call volume is desired

After All Consumers Are Migrated

• Update `aws-sdk-go-cache` README to indicate deprecation
• Archive the `keikoproj/aws-sdk-go-cache` repository on GitHub (issues are already disabled)