[jfx21u] RFR: 8304008: Update README.md and CONTRIBUTING.md for jfx update repos

[Kevin Rushforth]

Backport the changes to the README and CONTTRIBUTING guidelines for update releases. There are two commits: the first is a clean backport of the fix that went into jfx20u. The second is a simple substitution changing 20 to 21 in the two files.

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

Commit messages:
 - Change 20 --> 21
 - 8304008: Update README.md and CONTRIBUTING.md for jfx update repos

Changes: https://git.openjdk.org/jfx21u/pull/2/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx21u&pr=2&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304008
  Stats: 267 lines in 2 files changed: 1 ins; 254 del; 12 mod
  Patch: https://git.openjdk.org/jfx21u/pull/2.diff
  Fetch: git fetch https://git.openjdk.org/jfx21u.git pull/2/head:pull/2

PR: https://git.openjdk.org/jfx21u/pull/2