[jfx17u] RFR: 8273280: Update README.md and CONTRIBUTING.md for jfx17u

[Kevin Rushforth]

This PR updates [`README.md`](https://github.com/openjdk/jfx17u/blob/3082a845253dd00d46d36500abc42464459e59ed/README.md) and [`CONTRIBUTING.md`](https://github.com/openjdk/jfx17u/blob/3082a845253dd00d46d36500abc42464459e59ed/CONTRIBUTING.md) to be suitable for update releases. The two files were copied from jfx11u, with the only changes being to replace "11" with "17" in the two files.

-------------

Commit messages:
 - 8273280: Update README.md and CONTRIBUTING.md for jfx17u

Changes: https://git.openjdk.java.net/jfx17u/pull/1/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx17u&pr=1&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273280
  Stats: 237 lines in 2 files changed: 0 ins; 227 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx17u/pull/1.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx17u pull/1/head:pull/1

PR: https://git.openjdk.java.net/jfx17u/pull/1