docs: correct RC3 changelog — Java 17 baseline (lowered from 21)

RC3 wasn't housekeeping — it lowered the Java baseline from 21
(declared in RC1) back to 17 so the library can be consumed by
applications still on Java 17. The current build is still on Java 17.

Updated the RC3 entry to call this out as the headline change and
amended the 4.0.0 preamble so the Java baseline is stated correctly
(Java 17, not 21) instead of inheriting the now-superseded RC1 note.

Author: sonus21

docs/CHANGELOG.md

@@ -12,10 +12,11 @@ All notable user-facing changes to this project are documented in this file.
 
 {: .highlight}
 First stable 4.0.0 release. Targets Spring Boot 4.x and Spring Framework 7.x on
-Java 21. Promotes the RC6 line to GA — no functional changes versus RC6. See
-RC1 / RC2 below for the foundational Spring Boot 4 and Jackson 3 migration
-notes; RC4–RC6 below for the NATS backend, broker SPI, dashboard work, and
-middleware additions that build on top.
+Java 17 (lowered from the original Java 21 baseline in RC3). Promotes the RC6
+line to GA — no functional changes versus RC6. See RC1 / RC2 below for the
+foundational Spring Boot 4 and Jackson 3 migration notes; RC3 for the Java 17
+baseline change; RC4–RC6 below for the NATS backend, broker SPI, dashboard
+work, and middleware additions that build on top.
 
 ## Release [4.0.0.RC6] TBD
 
@@ -96,10 +97,16 @@ JetStream backend.
 
 ## Release [4.0.0.RC3] 14-Apr-2026
 
-{: .note}
-Housekeeping release. No user-facing code changes versus RC2 — version bump
-plus documentation tweaks (header refresh, dependabot bump for `addressable`
-in `/docs`).
+{: .highlight}
+Lowers the Java baseline from 21 back to 17.
+
+### Changes
+* **Java 17 baseline (was 21 in RC1/RC2)** — `languageVersion`,
+  `sourceCompatibility`, and `targetCompatibility` reverted from 21 to 17 so
+  the library can be consumed by applications still on Java 17. RC1's
+  "Java 21 baseline" note is superseded — 4.0.0 supports Java 17 and above.
+* Documentation tweaks (header refresh, dependabot bump for `addressable`
+  in `/docs`).
 
 ## Release [4.0.0.RC2] 24-Mar-2026