Fixes GRDynamicVariable in GemStone

• Added GRDynamicVariable class>>defaultValue to specify a default value for the dynamic variable.
• Removed Pharo 1.x and 2.x as well as Squeak 4.x from the baseline as we do no longer support them.

• Pharo6 compatible
• GRDynamicVariable to use DynamicVariable implementation when available in the host platform.
• Preview implementations for GRSmallDictionary and GROrderedMultiMap as GRSmallDictionary2 and GROrderedMultiMap2
• In Pharo, extensions to Behavior are now on TBehavior

Fixed a missing method: invalidUtf8 in GRPharoUtf8Codec

1.2.0 is a bugfix release but received a minor version number increase because it is the prerequisite version for Seaside 3.2.0 (the bindingOf: method added in Grease 1.2.0 is required for Seaside 3.2.0)