[jdk15u-dev] RFR: 8278997: Bump update version for OpenJDK: jdk-15.0.7

[Yuri Nesterenko]

Switch to 7-th release in 15-th release train. JBS number is in place.

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Commit messages:
 - 8278997: Bump update version for OpenJDK: jdk-15.0.7

Changes: https://git.openjdk.java.net/jdk15u-dev/pull/153/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk15u-dev&pr=153&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278997
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk15u-dev/pull/153.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk15u-dev pull/153/head:pull/153

PR: https://git.openjdk.java.net/jdk15u-dev/pull/153