JWK Set's HTTP client only overwrites and appends JWK to local cache during refresh

### Impact The project's provided HTTP client's local JWK Set cache should do a full replacement when the goroutine refreshes the remote JWK Set. The current behavior is to overwrite or append. This is a security issue for use cases that utilize the provided auto-caching HTTP client and where key removal from a JWK Set is equivalent to revocation.

Example attack scenario: 1. An attacker has stolen the private key for a key published in JWK Set. 2. The publishers of that JWK Set remove that key from the JWK Set. 3. Enough time has passed that the program using the auto-caching HTTP client found in `github.com/MicahParks/jwkset` v0.5.0-v0.5.21 has elapsed its `HTTPClientStorageOptions.RefreshInterval` duration, causing a refresh of the remote JWK Set. 4. The attacker is signing content (such as JWTs) with the stolen private key and the system has no other forms of revocation.

### Patches The affected auto-caching HTTP client was added in version `v0.5.0` and fixed in `v0.6.0`. Upgrade to `v0.6.0` or later.

### Workarounds The only workaround would be to remove the provided auto-caching HTTP client and replace it with a custom implementation. This involves setting the `HTTPClientStorageOptions.RefreshInterval` to zero (or not specifying the value). Upgrade to `v0.6.0` is advised.

### References Please see the tracking issue on GitHub for additional details: https://github.com/MicahParks/jwkset/issues/40