RFR: 8304956: Update KeyStore.getDefaultType​() specification to return pkcs12 as fallback

[Ben Perez]

Replaced "jks" with "pkcs12" in both the spec and fallback for `KeyStore.getDefaultType()`

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Commit messages:
 - removed jks as fallback keystore and replaced with pkcs12

Changes: https://git.openjdk.org/jdk/pull/15625/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15625&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304956
  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15625.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15625/head:pull/15625

PR: https://git.openjdk.org/jdk/pull/15625