`update_allows_fetch_and_merge` rejected as invalid rule type for branch-target `github_repository_ruleset` (regression in 6.11.0's CustomizeDiff validation)

### Expected Behavior
`update_allows_fetch_and_merge` should be accepted as a parameter of the `update` rule for a `branch`-target `github_repository_ruleset`, as documented, and as it worked prior to provider 6.11.0.

### Actual Behavior
`terraform plan` fails with:
```
Error: rule "update_allows_fetch_and_merge" is not valid for branch target; branch targets only support: [creation update deletion required_linear_history required_signatures pull_request required_status_checks non_fast_forward commit_message_pattern commit_author_email_pattern committer_email_pattern branch_name_pattern tag_name_pattern workflows code_scanning required_deployments merge_queue copilot_code_review]
```
This happens even though the same config previously worked and the setting ("Allow fork syncing" in the GitHub UI) is still active on the repository — GitHub's API itself accepts this parameter fine, so the current state already has `update_allows_fetch_and_merge = true` on that ruleset.

### Root cause
The `CustomizeDiff` validation added in v6.11.0 (#2958, `util_ruleset_validation.go`, `validateRules`/`branchTagOnlyRules`) iterates over every attribute set under `rules { }` and rejects any name that isn't in a hardcoded list of `github.RepositoryRuleType` values. `update_allows_fetch_and_merge` is not itself a rule type — it's a boolean *parameter* of the `update` rule type (mirrors `UpdateRuleParameters.UpdateAllowsFetchAndMerge` in go-github) — so it can never appear in that list and is always rejected once it's `true`.

### Terraform / Provider Version
- Terraform v1.14.8
- `integrations/github` v6.13.0 (reproduces on all versions ≥ 6.11.0; confirmed working on 6.6.0)

### Affected Resource
`github_repository_ruleset`

### Steps to Reproduce
```hcl
resource "github_repository_ruleset" "this" {
name = "main"
repository = "some-repo"
target = "branch"
enforcement = "active"

conditions {
ref_name {
include = ["refs/heads/main"]
exclude = []
}
}

rules {
update = true
update_allows_fetch_and_merge = true
}
}
```
`terraform plan` (no apply needed — the error is raised by `CustomizeDiff`, purely client-side).

### Additional context
Confirmed this is not fixable via `lifecycle { ignore_changes = [...] }` on that attribute — Terraform Core reverts ignored attributes to their prior-state value before invoking the provider's plan, and since the attribute is already `true` in state, it's fed into the same buggy validation on every plan regardless of whether it's ignored, present, or absent.