Update RightShift stderr reference for nint/nuint keywords

The grammar now includes 'nint' and 'nuint' as contextual keywords,
which changes the ANTLR error message token list.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: BillWagner

tools/GrammarTesting/Tests/Parsing/Samples/v6/RightShift/Reference/sample.stderr.txt

@@ -1,3 +1,3 @@
 line 10:12 no viable alternative at input '>'
-line 10:14 extraneous input '>' expecting {'_', '-', '--', '!', '..', '(', '&', '^', '+', '++', '~', 'add', 'alias', 'ascending', 'async', 'await', 'base', 'bool', 'by', 'byte', 'char', 'checked', 'decimal', 'delegate', 'descending', 'double', 'dynamic', 'equals', 'float', 'from', 'get', 'global', 'group', 'int', 'into', 'join', 'let', 'long', 'nameof', 'new', 'notnull', 'object', 'on', 'orderby', 'partial', 'remove', 'sbyte', 'select', 'set', 'short', 'sizeof', 'stackalloc', 'string', 'this', 'typeof', 'uint', 'ulong', 'unchecked', 'unmanaged', 'ushort', 'value', 'var', 'when', 'where', 'yield', 'default', 'null', 'true', 'false', '*', Simple_Identifier, Integer_Literal, Real_Literal, Character_Literal, String_Literal, Interpolated_Regular_String_Start, Interpolated_Verbatim_String_Start}
+line 10:14 extraneous input '>' expecting {'_', '-', '--', '!', '..', '(', '&', '^', '+', '++', '~', 'add', 'alias', 'ascending', 'async', 'await', 'base', 'bool', 'by', 'byte', 'char', 'checked', 'decimal', 'delegate', 'descending', 'double', 'dynamic', 'equals', 'float', 'from', 'get', 'global', 'group', 'int', 'into', 'join', 'let', 'long', 'nameof', 'new', 'nint', 'notnull', 'nuint', 'object', 'on', 'orderby', 'partial', 'remove', 'sbyte', 'select', 'set', 'short', 'sizeof', 'stackalloc', 'string', 'this', 'typeof', 'uint', 'ulong', 'unchecked', 'unmanaged', 'ushort', 'value', 'var', 'when', 'where', 'yield', 'default', 'null', 'true', 'false', '*', Simple_Identifier, Integer_Literal, Real_Literal, Character_Literal, String_Literal, Interpolated_Regular_String_Start, Interpolated_Verbatim_String_Start}
 line 11:8 no viable alternative at input '>'