RFR: 8278349: JarSigner javadoc example uses SHA-1

[Eirik Bjorsnos]

Please review this trivial PR which updates javadoc sample code in jdk.security.jarsigner.JarSigner to use SHA-256, SHA256withRSA instead of the now considered weak algorithms SHA-1, SHA1withDSA.

The sample code is also updated to use the new snippet syntax.

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Commit messages:
 - Replace use of weak algorithms SHA-1 and SHA1withDSA in JarSigner javadoc with SHA-256 and SHA256withRSA. Update to use @snippet syntax.

Changes: https://git.openjdk.org/jdk/pull/11992/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11992&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278349
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/11992.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11992/head:pull/11992

PR: https://git.openjdk.org/jdk/pull/11992