RFR: 8366232: JFR startup messages are shown with -Xlog:jfr+startup=warning

[Johannes Bechberger]

Only print the JFR startup messages when no or a < warning log level is set explicitly by the user.

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Commit messages:
 - Fix startup message printing

Changes: https://git.openjdk.org/jdk/pull/26957/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26957&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366232
  Stats: 66 lines in 8 files changed: 54 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/26957.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26957/head:pull/26957

PR: https://git.openjdk.org/jdk/pull/26957