[jfx23u] RFR: 8340954: Add SECURITY.md file

[Kevin Rushforth]

Clean backport of new `SECURITY.md` file to the `jfx23u` repo.

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Commit messages:
 - Backport 0dd0c794c3b08f816e7618026d5c90deaf952046

Changes: https://git.openjdk.org/jfx23u/pull/23/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx23u&pr=23&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340954
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx23u/pull/23.diff
  Fetch: git fetch https://git.openjdk.org/jfx23u.git pull/23/head:pull/23

PR: https://git.openjdk.org/jfx23u/pull/23