Update to sygus standard output for check-synth responses (#6521)

Update to sygus standard output for check-synth responses (#6521)

This PR does two things:
(1) It eliminates the ad-hoc implementation of printSynthSolutions and removes it from the API. Now, printing response to a check-synth command is done in a more standard way, using the API + symbol manager. This is analogous to recent refactoring to get-model.
(2) It updates cvc5's output in response to check-synth to be compliant with the upcoming sygus 2.1 standard. The standard has changed slightly: responses to check-synth are now closed in parentheses, mirroring the smt2 response to get-model.

It also removes the unused command GetSynthSolutionCommand.