[Tracking] Increase light-mode border contrast for Net11 editable controls

Related #14906

The visible light-mode bottom edge (the resting border affordance) does not meet WCAG 1.4.11 (non-text contrast, 3:1). Measured over a white field with Accessibility Insights it is 1.744:1 (#C4C4C4). WinUI's own field borders fall short of 1.4.11 as well.

Reaching 3:1 over white needs roughly #949494 or darker. PR #14919 raises the bottom edge to about 4.5:1, which is more prominent than the native WinUI border and matches Klaus's direction on #14906. The subtle side stroke is left at the WinUI value (about 1.06:1), on the underline-forward model where the bottom edge is the boundary affordance.

Open questions for @Olina-Zhang: confirm the exact WCAG-safe value for the bottom edge, whether the sides should also be brought to 3:1, or whether the team accepts the WinUI-style exception where the field is identified by other cues. The target depends on the field's BackColor; the values above are for the default white field. Dark-mode contrast is fine and out of scope.