Replace $t-gray-a-08 usage with suitable color primitive

From: !137738 (comment 1683175903)

A couple of instances of $t-gray-a-08 exist to leverage a light shade of gray that inverts for dark mode. These should be updated to suitable $gray-50/$gray-100 shades to ensure they are updated as expected. Border usage can be updated to $border-color to align with consistent styles throughout the platform.

Once updated the override in _dark.scss can be removed to ensure alpha shades are static between modes.

Usage examples:

• https://sourcegraph.com/search?q=context:global+repo:gitlab.com/gitlab-org/gitlab.*+%24t-gray-a-08&patternType=structural&sm=0&groupBy=repo
• !133709 (merged)
• !137347 (merged)
• !137738 (comment 1689259164)