fvConstraints: Remove 'Constraint' from constraint names

The fact that these names refer to constraints is clear in context, so
the name does not need to contain 'Constraint'.

Having 'Constraint' in the name is a historic convention that dates back
to when fvConstraints and fvModels were combined in a single fvOptions
interface. In this interface, disambiguation between sources and
constraints was necessary.

This change has been applied to the 'fixedValue' and 'fixedTemperature'
constraints, which were formerly named 'fixedValueConstraint' and
'fixedTemperatureConstraint', respectively.

The old names are still available for backwards compatibility.

tutorials/multiRegion/CHT/coolingSphere/templates/system/solid/fvConstraints

@@ -16,7 +16,8 @@ FoamFile
 /*
 fixedTemperature
 {
-    type            fixedTemperatureConstraint;
+    type            fixedTemperature;
+
     select          all;
 
     mode            uniform;

tutorials/multiRegion/CHT/reverseBurner/system/gas/fvConstraints

@@ -16,7 +16,7 @@ FoamFile
 
 ignition
 {
-    type            fixedTemperatureConstraint;
+    type            fixedTemperature;
 
     cellSet         ignition;