RFR: 8358099: PEM spec updates

[Anthony Scarpino]

Hi, I need a review of some PEM updates.  `PEMRecord.pem` is renamed to `content` to better describe that it is the base64 content and not including the header and footer. Additionally, `PEMRecord.getEncoded()` is removed and some javadoc clarifications for PEMEncoder and PEMDecoder.  Thanks.

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Commit messages:
 - initial

Changes: https://git.openjdk.org/jdk/pull/25620/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25620&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358099
  Stats: 67 lines in 6 files changed: 1 ins; 20 del; 46 mod
  Patch: https://git.openjdk.org/jdk/pull/25620.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25620/head:pull/25620

PR: https://git.openjdk.org/jdk/pull/25620