RFR: 8338700: AttributeMapper type parameter should be bounded by Attribute

[Chen Liang]

A minor oversight in AttributeMapper type parameter bounds, that it should be bounded by Attribute. Only real impact is in BoundAttribute.readAttributes where a cast is now omitted, as other sites of access like Attribute::attributeMapper has already bounded the returned type argument.

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Commit messages:
 - 8338700: AttributeMapper type parameter should be bounded by Attribute

Changes: https://git.openjdk.org/jdk/pull/20668/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20668&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338700
  Stats: 6 lines in 5 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20668.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20668/head:pull/20668

PR: https://git.openjdk.org/jdk/pull/20668