RFR: 8255713: JavaFX build should discover Visual Studio compiler on system

[Kevin Rushforth]

We currently hard-code the default version of Visual Studio in both `win.gradle` and `.github/workflows/submit.yml`. This hard-coding of the specific version of MSVC is fragile, particularly for GitHub action builds. The last time Microsoft changed the environment, our Windows builds started failing (because we hard-coded it). I temporarily bumped it to the then-current version as a fix for [JDK-8256686](https://bugs.openjdk.java.net/browse/JDK-8256686), and filed this bug -- [JDK-8255713](https://bugs.openjdk.java.net/browse/JDK-8255713) -- as a followup. It's failing again because of another update to the GitHub actions environment, so it's time to fix it to discover the latest version rather than hardcode it.

NOTE to reviewers: This will need to be tested with local and CI builds as well as GitHub actions, since there are changes in the `win.gradle` script (and associated `genVSproperties.bat` script).

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Commit messages:
 - 8255713: JavaFX build should discover Visual Studio compiler on system

Changes: https://git.openjdk.java.net/jfx/pull/427/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=427&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255713
  Stats: 25 lines in 3 files changed: 17 ins; 2 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/427.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/427/head:pull/427

PR: https://git.openjdk.java.net/jfx/pull/427