Revert deprecated `PaymentButton` colors

[stechiu]

Reason for changes

Design leadership is working on re-branding PayPal. We do not want to make any branding changes before this major update happens.

Revert the changes that deprecated the button colors. Previously merged PR https://github.com/paypal/paypal-ios/pull/231

Summary of changes

• Undeprecated colors blue, dark blue, silver, black

Checklist

• [x] Added a changelog entry

Authors

List GitHub usernames for everyone who contributed to this pull request.

• @stechiu

[stechiu]

It looks like we have .gold defined in a few other places that I assume we also will want to remove: https://github.com/search?q=repo%3Apaypal%2Fpaypal-ios%20gold&type=code

Gold was around before so I left them in