RFR: Using backports for stabilization repositories

[Erik Helin]

Hi all,

please review this patch that replaces the documentation on the "forward ports" process with the [recently adopted](https://mail.openjdk.org/pipermail/jdk-dev/2023-May/007889.html) "backports" process. The changes are fairly small since the guide already contains great documentation on backports, I just had to adopt it slightly to also cover stabilization repositories.

Thanks!
Erik

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Commit messages:
 - Using backports for stabilization repositories

Changes: https://git.openjdk.org/guide/pull/105/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=105&range=00
  Stats: 19 lines in 2 files changed: 0 ins; 15 del; 4 mod
  Patch: https://git.openjdk.org/guide/pull/105.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/105/head:pull/105

PR: https://git.openjdk.org/guide/pull/105