RFR: 8373538: Migrate all tests to null-safe "SimpleSSLContext" methods [v3]

[Volkan Yazici]

> 1. [JDK-8372661] introduced null-safe static factory methods to `SimpleSSLContext`
> 2. [JDK-8373515] migrated `test/jdk/java/net/httpclient/` to these new methods
> 3. [JDK-8373537] migrated `test/jdk/com/sun/net/httpserver/` to these new methods
> 4. This PR migrates all remaining `SimpleSSLContext` usages, *and* removes nullable `SimpleSSLContext` factory methods
> 
> [JDK-8372661]: https://bugs.openjdk.org/browse/JDK-8372661
> [JDK-8373515]: https://bugs.openjdk.org/browse/JDK-8373515
> [JDK-8373537]: https://bugs.openjdk.org/browse/JDK-8373537

Volkan Yazici has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains nine commits:

 - Update copyright years
 - Migrate forgotten `ClearTextServerSSL`
 - Merge remote-tracking branch 'upstream/master' into simpleSslRest
 - Remove wildcard imports
 - Merge remote-tracking branch 'upstream/master' into simpleSslRest
 - Restore all non-`httpclient`, non-`httpserver` changes in `test/`
 - Make sure `get()` returns the same instance
 - Reverted all changes and only kept `SimpleSSLContext` enhancements
 - Overhaul `SimpleSSLContext` and its usages

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Changes: https://git.openjdk.org/jdk/pull/28905/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28905&range=02
  Stats: 77 lines in 12 files changed: 4 ins; 39 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/28905.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28905/head:pull/28905

PR: https://git.openjdk.org/jdk/pull/28905