RFR (CSR) JDK-8245399 Remove addition preview adornment from String::formatted

[Jim Laskey]

Please review this change to remove the preview heading from the javadoc of String::formatted. This also updates the @since to 15.

Thank you.

Cheers,

-- Jim


csr: https://bugs.openjdk.java.net/browse/JDK-8245399 <https://bugs.openjdk.java.net/browse/JDK-8245399>
jbs: https://bugs.openjdk.java.net/browse/JDK-8245398 <https://bugs.openjdk.java.net/browse/JDK-8245398>