change the name RSA256 to PS256

[EvergreenHZ]

hi, the COSE_Sign1 signing algorithm should be PS256 instead of RSA256, which may be confusing.
the 'alg' label value is -37, check here and here.

[goebelUB]

Where exactly did you see this? Just grepping for it I fail to find any occurrence of neither PS256 nor RSA256 in the Kotlin SDK.

[TianpingHsu]

sorry, it's private const val ALG_RSA_256 = "RS256" in Jwt.kt.