RFR: 8340899: Remove wildcard bound in PositionWindows.positionTestWindows

[Alexey Ivanov]

Removes the wildcard bound from `PassFailJFrame.PositionWindows.positionTestWindows` so that implementing the interface is simpler as one don't have to remember to declare the `testWindows` parameter as `List<? extends Window>` — just use `List<Window>`.

It is a backwards compatible change, all the tests which use `List<? extends Window>` continue to compile successfully.

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Commit messages:
 - 8340899: Remove wildcard bound in PositionWindows.positionTestWindows

Changes: https://git.openjdk.org/jdk/pull/21181/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21181&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340899
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21181.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21181/head:pull/21181

PR: https://git.openjdk.org/jdk/pull/21181