fix: #3624257 Fix incorrect route provider return types from ?\Serializable to ?Route on 7.x

Closes #3624257

Backports #3594984 to 7.x.

Changes:

• Replaces the incorrect ?\Serializable return types with ?Route in the four Cloud route providers.

Validation:

• Ran php -l on every changed PHP file.
• Ran git diff --check.