[jdk17u-dev] RFR: 8343474: [updates] Customize README.md to specifics of update project

[Goetz Lindenmaier]

I want to keep the README in sync with 21.

Adapted all mention of 21 to 17.

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Commit messages:
 - Backport 22d5e0d1f8849410abe40165b58f45f5e4293884

Changes: https://git.openjdk.org/jdk17u-dev/pull/3040/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3040&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343474
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/3040.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3040/head:pull/3040

PR: https://git.openjdk.org/jdk17u-dev/pull/3040