Document the upcoming deprecation and eventual removal of the Validating Webhook

[robscott]

With the transition to CEL becoming a reality in v0.8.0, we are going to begin the process of spinning down support for the validating webhook. I expect the timeline will go as follows:

v0.8.0:

• CEL validation is included in all CRDs with only one missing feature from webhook validation: CEL Validation: HTTPRoute Case-Insensitive Header Name Uniqueness #2277
• Installation of the webhook is still recommended, but release notes state that this will be the last release it is recommended for
• Implementations are strongly encouraged to run validation code from webhook on the controller-side, primarily to cover the gap described in CEL Validation: HTTPRoute Case-Insensitive Header Name Uniqueness #2277

v1.0:

• Validating webhook is formally deprecated, installation is only recommended when using Gateway API on clusters <= 1.24
• Implementations are required to cover CEL Validation: HTTPRoute Case-Insensitive Header Name Uniqueness #2277 with controller-side validation (likely reusing the validation code from the webhook)

v1.1:

• The webhook is removed from the codebase
• Validation code remains as a library for controllers to run, covering any gaps in validation that can't be covered with CEL (currently limited to CEL Validation: HTTPRoute Case-Insensitive Header Name Uniqueness #2277)