Update GitLab payloads

[colinodell]

This MR makes some changes and improvements to GitLab payloads:

• Adds support for more useful fields in Changes
• Deprecates the redundant Assignee and Reviewers types in favor of Users, which already has the same properties
• Bumps the minimum Go version to 1.18 so we can use generics to greatly simplify the definition of Changes fields

[coveralls]

coverage: 88.229% (-0.5%) from 88.765%
when pulling fbf0071 on colinodell:codell-gitlab-payloads
into 53694f8 on go-playground:master.