USWDS-Site: Major version updates #3279

Major version update opportunities identified via npm outdated. Each item crosses a major version boundary and may include breaking changes, so each should be reviewed/tested individually.

• @18f/identity-stylelint-config 2.0.0 → 4.1.0
• concurrently 8.2.2 → 10.0.4
• del 6.1.1 → 8.0.1
• eslint-config-prettier 9.1.0 → 10.1.8
• gulp-postcss 9.0.1 → 10.0.0
• gulp-sourcemaps 2.6.5 → 3.0.0
• jquery 3.7.1 → remove
• pa11y-ci 2.4.2 → 4.1.1
• prettier 2.8.8 → 3.9.6
• stylelint 15.11.0 → 17.14.1