docs: Add the maximum implemented Vulkan API version in 19.2 rel notes

Currently, Vulkan 1.1.

Cc: 19.2 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit 41b0e0d7)