fix: myinfo encryptedAndSignedPersona to use enc A256GCM

[lwinxp]

Problem

1. In myinfo docs (Developers_FAQ.pdf), noted use of A256GCM

2. Q15 of https://api.singpass.gov.sg/library/myinfo/developers/FAQ

3. current product integration with myinfo test api is also using A256GCM and it is working

Solution

1. Update A128CBC-HS256 to A256GCM

[cflee]

I double-checked the TUO v3.2.6 spec that I have on hand (in case it differs from v4 as linked above) and it does say A256GCM as well. Looks like it was left like this in #563 for backwards compatibility reasons, but I think there's a case to be made that this is a bugfix (instead of a breaking change) as any client strictly enforcing A128CBC-HS256 instead of A256GCM is doing it wrong anyway.

[lwinxp]

thanks for the review and finding more info on this

the pr title and commit message have been changed to fix