Process enhancements to maximize success of Dogfooding initiatives

Why is this change being made?

The purpose of this MR is to make it more clear that Dogfooding initiatives will be more successful if internal customers are included early on in the feature development process so that their needs are considered up front. Oftentimes, when internal teams get asked to Dogfood a feature, the feature set has already been established and doesn't address the needs of internal teams, making Dogfooding difficult as discussed here: https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11511#note_436288226. This MR addresses this issue by adding a step for PMs to 'seek internal feedback' during the validation phase of the product development flow.

Author Checklist

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