[15u] RFR: 8262541: Bump update version for OpenJDK: jdk-15.0.3

[Yuri Nesterenko]

Hi,

we have several fixes presumably targeted to April release of jdk15u
which should be 15.0.3,  so I need to bump the version.
Release value 15.0.3 is defined in JBS.
Expected release date is 2021-04-20

I'll push the change after hgupdater is switched to it for
 http://hg.openjdk.java.net/jdk-updates/jdk15u/.

Webrev: http://cr.openjdk.java.net/~yan/8262541/webrev.0/

Thanks,
--yan