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

[Johan Vos]

On Wed, 19 Jul 2023 17:13:00 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> 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.

all good

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Marked as reviewed by jvos (Reviewer).

PR Review: https://git.openjdk.org/jfx21u/pull/2#pullrequestreview-1537709823