<AWT Dev> RFR: 8254850: Update terminology in java.awt.GridBagLayout source code comments

[Phil Race]

This is part of an effort in the JDK to replace archaic/non-inclusive words with more neutral terms.
See JDK-8253315 for details.
 
This updates a few comments in GridBagLayout to use the terms parent and child which are
sufficiently descriptive and consistent with lots of other usages.

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Commit messages:
 - Update terminology in java.awt.GridBagLayout source code comments

Changes: https://git.openjdk.java.net/jdk/pull/1784/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1784&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254850
  Stats: 10 lines in 1 file changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1784.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1784/head:pull/1784

PR: https://git.openjdk.java.net/jdk/pull/1784