Change the Authorization header to use Bearer (#748)

While Maven Central does support the UserToken header they recommend using the standard Bearer value, as future versions of the API might drop support for UserToken

https://central.sonatype.org/publish/publish-portal-api/#authentication-authorization

central-portal/src/main/kotlin/com/vanniktech/maven/publish/portal/SonatypeCentralPortalOkHttpInterceptor.kt

@@ -12,7 +12,7 @@ internal class SonatypeCentralPortalOkHttpInterceptor(
     val requestBuilder = chain.request().newBuilder()
 
     requestBuilder.addHeader("Accept", "application/json") // request json by default, XML is returned else
-    requestBuilder.addHeader("Authorization", "UserToken $usertoken")
+    requestBuilder.addHeader("Authorization", "Bearer $usertoken")
     requestBuilder.addHeader("User-Agent", "$userAgentName/$userAgentVersion")
 
     return chain.proceed(requestBuilder.build())