Inconsistent handling of YEAR substitution across generators

### Background

https://github.com/kubernetes-sigs/controller-tools/pull/544 added a `year` option to generator commands (`object`, `crd`, `rbac`, and `webhook`), allowing callers to substitute a `YEAR` placeholder in boilerplate header files with a specific value.

### Problem

The `applyconfiguration` generator is missing the `year` option and unconditionally replaces `YEAR` using `time.Now().UTC().Year()`, leading to inconsistent behaviour.

The full call chain is:

1. [`controller-tools/pkg/applyconfiguration/gen.go#L215`](https://github.com/kubernetes-sigs/controller-tools/blob/fbbff17babfacd1ebbbb7a7842bb5378179dadf5/pkg/applyconfiguration/gen.go#L215)
2. [`controller-tools/pkg/applyconfiguration/gen.go#L293`](https://github.com/kubernetes-sigs/controller-tools/blob/fbbff17babfacd1ebbbb7a7842bb5378179dadf5/pkg/applyconfiguration/gen.go#L293)
3. [`code-generator/cmd/applyconfiguration-gen/generators/targets.go#L60`](https://github.com/kubernetes/code-generator/blob/v0.35.0/cmd/applyconfiguration-gen/generators/targets.go#L60)
4. [`gengo/v2/execute.go#L59`](https://github.com/kubernetes/gengo/blob/5ee0d033ba5bcb073f0e69c32057520b82d75ccb/v2/execute.go#L59)

### Proposed Solution

Add a optional `year` option to the `applyconfiguration` generator, which when set, will take precedence. This change would be backwards compatible, as if the option is not specified, the current logic stays as it currently is.

Additionally, the other generators could have same logic, of defaulting to the current year as a subsitution for the string "YEAR" if not overridden by the option.