Remove `external_links` when DDEV v1.24.9 is out

[stasadev]

This add-on should:

• Remove existing docker-compose.selenium-chrome_extras.yaml when DDEV v1.24.9 is out.
• Add ddev_version_constraint: '>= v1.24.9'
• Remove "Run ddev add-on get ddev/ddev-selenium-standalone-chrome after changes to" from README.md

Reference:

• feat(docker): auto HTTP/S communication via network aliases instead of external_links ddev#7642