Refactor api error

## Problem 1: The presence of skyerr and APIError ✅

Remove skyerr and use APIError

## Problem 2: APIError is not errors friendly

For example, APIError does not Unwrap, nor it supports [errors.Is](http://errors.Is). This causes us to use custom functions like apierrors.IsKind or IsRateLimitErrorWithBucketName to compare error. Example of bugs: [https://github.com/authgear/authgear-server/commit/6b660e8eb60572ddc5b1d089a912be051dc15823](https://github.com/authgear/authgear-server/commit/6b660e8eb60572ddc5b1d089a912be051dc15823)

## Problem 3: some features in errorutil duplicate with errors

For example, errorutil.Unwrap, which can be replaced by [errors.Is](http://errors.Is) and [errors.As](http://errors.As). errorutil.WithSecondaryError, which can be replaced by errors.Join.

Start by tracing usages of skyerr, APIError, and errorutil across the repository, then review the linked commit for the stated comparison bug. The work is done when skyerr is removed, APIError supports standard error inspection, and duplicated errorutil features are addressed without breaking callers.

Written by the indexing model from the issue text.