refactor(design, daffio)!: update daff-gray palette name to daff-neutral

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Palette is currently named daff-gray. The gray wording is confusing because the palette does not just have gray colors.

Fixes: #1894

What is the new behavior?

Update palette name to daff-neutral which will allow the palette to be defined with more flexibility.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Other information