RFR: 8274321: Standardize values of @since tags in javax.lang.model

[Pavel Rappo]

This changes values of four `@since` tags from 8 to 1.8, which is predominantly used to refer to JDK 1.8.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/5699/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5699&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274321
  Stats: 6 lines in 2 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5699.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5699/head:pull/5699

PR: https://git.openjdk.java.net/jdk/pull/5699