fix: correct minimum nvc version for coverage support

Author: CharrierTim

vunit/sim_if/nvc.py

@@ -92,7 +92,7 @@ def __init__(  # pylint: disable=too-many-arguments
         (major, minor) = self.determine_version(prefix)
         self._supports_jit = major > 1 or (major == 1 and minor >= 9)
         self._ieee_warnings_global = major > 1 or (major == 1 and minor >= 16)
-        self._supports_coverage_merge = major > 1 or (major == 1 and minor >= 14)
+        self._supports_coverage_merge = major > 1 or (major == 1 and minor >= 15)
 
         if self.use_color:
             environ["NVC_COLORS"] = "always"