[jdk15u-dev] RFR: 8264255: Bump update version for OpenJDK: jdk-15.0.4

[Yuri Nesterenko]

Once the version number 15.0.4 is created in JBS and .jcheck/conf  is updated by 8264256, we should switch the current version number in make/autoconf/version-numbers

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Commit messages:
 - 8264255: Bump update version for OpenJDK: jdk-15.0.4

Changes: https://git.openjdk.java.net/jdk15u-dev/pull/2/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk15u-dev&pr=2&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264255
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk15u-dev/pull/2.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk15u-dev pull/2/head:pull/2

PR: https://git.openjdk.java.net/jdk15u-dev/pull/2