RFR: 8314940: Use of NIO in JDKs Metrics implementation causes issues in GraalVM

[Severin Gehwolf]

Please review this rather trivial fix to not use `nio` in `CgroupUtil`, part of the
JDK's Metrics API. The primary motivating factor is that it allows one to use the
JDK's version of `Metrics` in GraalVM. See the bug for details as to why this is
needed.

Testing:
- [x] GraalVM builds with/without the fix and the reproducer (fails before/works after)
- [x] `jdk/internal/platform` jtreg tests on Linux x86_64 (cgv1).
- [ ] GHA - still running

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Commit messages:
 - 8314940: Use of NIO in JDKs Metrics implementation causes issues in GraalVM

Changes: https://git.openjdk.org/jdk/pull/15416/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15416&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314940
  Stats: 29 lines in 1 file changed: 11 ins; 14 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15416.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15416/head:pull/15416

PR: https://git.openjdk.org/jdk/pull/15416