RFR: 8274525: Replace uses of StringBuffer with StringBuilder in java.xml

[Andrey Turbanov]

StringBuffer is a legacy synchronized class. There are more modern alternatives which perform better:
1. Plain String concatenation should be preferred
2. StringBuilder is a direct replacement to StringBuffer which generally have better performance

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Commit messages:
 - [PATCH] Cleanup usages of StringBuffer in java.xml

Changes: https://git.openjdk.java.net/jdk/pull/5759/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5759&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274525
  Stats: 32 lines in 5 files changed: 0 ins; 11 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5759.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5759/head:pull/5759

PR: https://git.openjdk.java.net/jdk/pull/5759