RFR: 8263759: Update boot JDK to 15.0.2

[Kevin Rushforth]

Simple fix to updated the boot JDK version to 15.0.2. Tested locally, and with the GitHub Actions build, both of which passed.

-------------

Commit messages:
 - 8263759: Update boot JDK to 15.0.2

Changes: https://git.openjdk.java.net/jfx/pull/431/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=431&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263759
  Stats: 11 lines in 2 files changed: 0 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jfx/pull/431.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/431/head:pull/431

PR: https://git.openjdk.java.net/jfx/pull/431