SI-8022 Backwards compatibility for Regex#unapplySeq

The change in ce1bbfe / SI-6406 introduced overloads of
`unapplySeq` with wider static and dynmaic result types
than the now-deprecated alternative that accepted `Any`.

This is subtly source incompatible and the change was noticed
in Specs2.

This commit uses `List` as the static and runtime type for
the new overloads.

For consistency, the same is done for the new method added
in SI-7737 / 93e9623.