[jdk15u-dev] RFR: 8264256: Update version .jcheck/conf in jdk15u-dev to be 15.0.4

[Yuri Nesterenko]

Instruct updater script to use 15.0.4 release version from now on. 
Should integrate that no sooner than the version is created in JBS.

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Commit messages:
 - 8264256: Update version .jcheck/conf in jdk15u-dev to be 15.0.4

Changes: https://git.openjdk.java.net/jdk15u-dev/pull/1/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk15u-dev&pr=1&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264256
  Stats: 28 lines in 1 file changed: 26 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk15u-dev/pull/1.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk15u-dev pull/1/head:pull/1

PR: https://git.openjdk.java.net/jdk15u-dev/pull/1