RFR: 8264029: Replace uses of StringBuffer with StringBuilder in java.base

[Andrey Turbanov]

Found by IntelliJ IDEA inspection `Java | Java language level migration aids | Java 5 | 'StringBuffer' may be 'StringBuilder'`
As suggested in https://github.com/openjdk/jdk/pull/1507#issuecomment-757369003 I've created separate PR for module `java.base`
Similar cleanup in the past - https://bugs.openjdk.java.net/browse/JDK-8041679

-------------

Commit messages:
 - [PATCH] Replaces usages of StringBuffer with StringBuilder where appropriate in java.base

Changes: https://git.openjdk.java.net/jdk/pull/2922/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2922&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264029
  Stats: 12 lines in 3 files changed: 0 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2922.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2922/head:pull/2922

PR: https://git.openjdk.java.net/jdk/pull/2922