RFR: 8360131: Remove use of soon-to-be-removed APIs by CTW framework

[David Beaumont]

Migrate the CWT framework to use only supported JRT file system access for fetching class bytes.
This avoids accessing APIs in ImageReader which are scheduled to be removed as part of preview mode class support in Valhalla (essentially these APIs are "too low level" and expose semantics that are incompatible with supporting preview classes in Valhalla).

This will be a further change to this code when the preview mode work goes in, but this will be limited to how the file system is opened (with or without preview mode).

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Commit messages:
 - Copyright update.
 - Remove comment.
 - Move CWT test away from soon-to-be-deleted ImageReader APIs.

Changes: https://git.openjdk.org/jdk/pull/25916/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25916&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360131
  Stats: 60 lines in 2 files changed: 32 ins; 4 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/25916.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25916/head:pull/25916

PR: https://git.openjdk.org/jdk/pull/25916