RFR: 8364954: Migrate CleaningThread in BufferCleaner to InnocuousThread

[Liam Miller-Cushon]

This migrates a `Thread` introduced in [JDK-8344332](https://bugs.openjdk.org/browse/JDK-8344332) to use `InnocuousThread`, similar to the changes in [JDK-8345432](https://bugs.openjdk.org/browse/JDK-8345432) to migrate other uses of `Thread` in `java.nio` to `InnocuousThread`.

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Commit messages:
 - 8364954: Migrate CleaningThread in BufferCleaner to InnocuousThread

Changes: https://git.openjdk.org/jdk/pull/26665/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26665&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364954
  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/26665.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26665/head:pull/26665

PR: https://git.openjdk.org/jdk/pull/26665