[8u] RFA: 8162572: Update License Header for all JAXP sources

[Alex Kashchenko]

Hi,

JDK-8162572 is not public, so requesting the approval here:

Fix request (8u)

Backport is requested because it can make it easier to backport
subsequent JAXP parity
issues to 8u (don't have a comprehensive list, intend to proceed with
JDK-8163121 first).

Review approval:

https://mail.openjdk.java.net/pipermail/jdk8u-dev/2021-October/014353.html

-- 
-Alex