[ASTERIXDB-3711]Fix README JDK recommendation

- user model changes: yes
- storage format changes: no
- interface changes: no

Details: Building master actually requries JDK 21, not 17.

Change-Id: I434142a9b9cd2d1fc941b1e1f250ef7403a813ec
Reviewed-on: https://asterix-gerrit.ics.uci.edu/c/asterixdb/+/20999
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Ian Maxon <imaxon@apache.org>
Reviewed-by: Suryaa Charan Shivakumar <sshiv012@ucr.edu>
Integration-Tests: Jenkins <jenkins@fulliautomatix.ics.uci.edu>

README.md

diff --git a/README.md b/README.md
index 2034914..c5f407d 100644
--- a/README.md
+++ b/README.md
@@ -57,7 +57,7 @@
 * A Unix-ish environment (Linux, OS X, will all do).
 * git
 * Maven 3.3.9 or newer.
-* JDK 17 or newer.
+* JDK 21 or newer.
 * Python 3.11+ with pip and venv
 
 Instructions for building the master: