RFR: 8276142: Update gradle to version 7.3

[Kevin Rushforth]

This bumps the version of gradle used to build JavaFX to 7.3 (from the current 7.0.1). Among other things, gradle 7.3 adds support for JDK 17, so this is a prerequisite to update the boot JDK to JDK 17.x -- see [JDK-8276144](https://bugs.openjdk.java.net/browse/JDK-8276144). This will not affect the minimum version of gradle needed to build JavaFX, which remains at 6.3 (although that version requires an even older boot JDK than we currently use, so is not recommended).

I included a `gradle/UPDATING.txt ` file to document the steps needed to update to a new version of gradle.

NOTE: The boot JDK remains at 16.0.2 with this PR, but the intent is to update the boot JDK soon after this is integrated. To that end, I have created PR #670 as a Draft PR to bump the boot JDK to 17.0.1. It necessarily includes the fix from _this_ PR to update gradle to 7.3, otherwise the GitHub Actions build would have failed. You can look at the test results from that Draft PR to see that JDK 17.0.1 works when buliding with gradle 7.3.

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Commit messages:
 - 8276142: Update gradle to version 7.3

Changes: https://git.openjdk.java.net/jfx/pull/671/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=671&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276142
  Stats: 227 lines in 7 files changed: 102 ins; 49 del; 76 mod
  Patch: https://git.openjdk.java.net/jfx/pull/671.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/671/head:pull/671

PR: https://git.openjdk.java.net/jfx/pull/671