Fix PersistenceThresholdFilter UoM conversion

Use `toUnitRelative()` instead of `toUnit()` for delta (the difference
of old and current state). In particular for temperatures an offset
of different units (like °C and K) should be ignored when considering
the delta.

Signed-off-by: Christoph Gehrke <cgdev@duck.com>

Author: ea21fd6

bundles/org.openhab.core.persistence/src/main/java/org/openhab/core/persistence/filter/PersistenceThresholdFilter.java

@@ -104,7 +104,7 @@ public boolean apply(Item item) {
                     }
                 } else if (!unit.isBlank()) {
                     // consider unit only if not relative threshold
-                    delta = delta.toUnit(unit);
+                    delta = delta.toUnitRelative(unit);
                     if (delta == null) {
                         throw new UnconvertibleException("");
                     }