Migrate gl-text-gray-300 -> gl-text-disabled

What does this MR do and why?

Migrates these like for like classes: gl-text-gray-300 -> gl-text-disabled

This MR does create some visual changes where we will use disabled text (gray-400) in light mode in place of gray-300

Please note, this is part Phase 1 (Large-Scale, Low-Risk Migrations) and as it is replacing a "like for like" class, it does not require manual validation.

Here is a link to the project page in groupdesign system handbook page: https://handbook.gitlab.com/handbook/engineering/development/dev/foundations/design-system/#design-token-migration-strategy

(the handbook page is not yet merged, here is the MR: gitlab-com/content-sites/handbook!10696 (merged))

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Related to #503083