RFR: 8313983: jmod create --target-platform should replace existing ModuleTarget attribute

[Adam Sotona]

ModuleTarget and ModuleResolution attributes were flagged as 'allow multiple' in the Classfile API.
This patch removed the flags and allows at most one instance of each attribute.

Please review.

Thanks,
Adam

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Commit messages:
 - 8313983: jmod create --target-platform should replace existing ModuleTarget attribute

Changes: https://git.openjdk.org/jdk/pull/15514/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15514&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313983
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15514.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15514/head:pull/15514

PR: https://git.openjdk.org/jdk/pull/15514