migrations: prevent accidentally using unversioned migrations instead of `VersionedMigration` (#3835)

closes #1324 

#### Problem
Currently, it is possible to accidentally use inner unversioned
migration instead of `VersionedMigration` since both implement
`OnRuntimeUpgrade`.

#### Solution

With this change, we make it clear that value of `Inner` is not intended
to be used directly. It is achieved by bounding `Inner` to new trait
`UncheckedOnRuntimeUpgrade`, which has the same interface (except
`unchecked_` prefix) as `OnRuntimeUpgrade`.

#### `try-runtime` functions

Since developers can implement `try-runtime` for `Inner` value in
`VersionedMigration` and have custom logic for it, I added the same
`try-runtime` functions to `UncheckedOnRuntimeUpgrade`. I looked for a
ways to not duplicate functions, but couldn't find anything that doesn't
significantly change the codebase. So I would appreciate If you have any
suggestions to improve this

cc @liamaharon @xlc 

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