Use for_each in resource github_repository_webhook

### Describe the Feature

Use the for_each instead of count to make the `github_repository_webhook` list more stable

### Expected Behavior

When deleting a git repo from the list, only one element is affected.

### Use Case

Currently, when I delete one item in the middle of my `var.github_repositories`, the items after it will be desstoyed and recreated to shift one position up.

### Describe Ideal Solution

Using for_each with maps will make it more stable.

### Alternatives Considered

_No response_

### Additional Context

_No response_