RFR: 8305814: Update Xalan Java to 2.7.3

[Joe Wang]

Xalan 2.7.3: merge minor changes from the upstream project.

Test: existing XML tests pass

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Commit messages:
 - 8305814: Update Xalan Java to 2.7.3

Changes: https://git.openjdk.org/jdk/pull/16532/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16532&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305814
  Stats: 66 lines in 4 files changed: 19 ins; 22 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/16532.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16532/head:pull/16532

PR: https://git.openjdk.org/jdk/pull/16532