RFR: 8233160: Add java.vendor.url.bug to list of recognized standard system properties

[Jaikiran Pai]

Can I please get a review of this change which proposes to address https://bugs.openjdk.org/browse/JDK-8233160?

It has been noted in https://bugs.openjdk.org/browse/JDK-8232753 that:

> The java.vendor.url.bug property has been defined by every Sun/Oracle JDK going all the way back to JDK 5 (and possibly earlier; JDK 5 is the oldest release that I can still run on my development machine). Yet, it's never been specified.

The OpenJDK builds too by default set a value for this system property.

The commit in this PR updates the javadoc of `System.getProperties()` to include this system property in the list of specified properties. Additionally, the `test/jdk/java/lang/System/PropertyTest.java` test has been updated to verify that this property is indeed available in the Properties returned by `System.getProperites()`.

I'll create a CSR shortly for this change.

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Commit messages:
 - 8233160: Add java.vendor.url.bug to list of recognized standard system properties

Changes: https://git.openjdk.org/jdk/pull/15504/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15504&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8233160
  Stats: 4 lines in 2 files changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15504.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15504/head:pull/15504

PR: https://git.openjdk.org/jdk/pull/15504