fix: remove 'pro' plan in billing copy

[AdityaHegde]

1. Old Pro plan is no longer in use, so update to remove it.
2. Add seat price to trial plan upgrade.
3. Move some text to i18n + spanish translation.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!