Switch AscertainedForParsimonyUninformativeFilteredAlignment to spec FilteredAlignment

beast.base.evolution.alignment.FilteredAlignment is @deprecated in
beast3 and replaced by beast.base.spec.evolution.alignment.FilteredAlignment.
The spec class still extends Alignment (the legacy StateNode, not
itself deprecated), so all the inherited fields the override relies on
— isAscertainedInput, excludefromInput, excludetoInput,
setupAscertainment() — remain available unchanged. Only the import
and extends clause needed updating.

mvn test: 8/8 pass.

Found via the beast3-migration analyzer: the only remaining Java
migration item for morph-models.

Author: alexeid

src/main/java/morphmodels/evolution/alignment/AscertainedForParsimonyUninformativeFilteredAlignment.java

@@ -1,7 +1,7 @@
 package morphmodels.evolution.alignment;
 
 import beast.base.core.Input;
-import beast.base.evolution.alignment.FilteredAlignment;
+import beast.base.spec.evolution.alignment.FilteredAlignment;
 
 import java.util.HashSet;
 import java.util.Set;