Deprecate secret "PermitDowngrade" flag

The secret "PermitDowngrade" flag should be deprecated (removed from IPM codebase).
This is because by allowing users to downgrade without making them uninstall the module first leads to more complications than good.
For example, if a module was at 1.1.0 and the install command is used to downgrade to 1.0.0, classes that were added in 1.1.0 will not be deleted, and will linger after the "downgrade".
Moving forward, if an user wants to downgrade a module, they must first run the uninstall command, and then run the install command with their desired version.

Work involved:
- Search for all instances of "PermitDowngrade" in the IPM code base and remove them.
- Test to make sure existing behavior has not changed.
- If documentation exists for the PermitDowngrade flag, remove it from all sources (Github Wiki, Stack Overflow, etc.) or at least update to say deprecated.