docs+ci: scrub remaining JDK 21 references

JPDFium's published artifacts target JVM 25 and Stirling-PDF's main
sources are now compiled with sourceCompatibility = VERSION_25, so JDK
21 is no longer supported. Update header comments and developer-facing
docs to reflect the hard floor.

- backend-build.yml header: 'JDK 21/25 \xc3\x97 spring-security' -> 'JDK 25 \xc3\x97 spring-security'
- AGENTS.md Important Notes: minimum 21 -> requires 25
- DeveloperGuide.md prerequisites + setup steps: drop 'or later, 25 recommended'
  language, say 'JDK 25' outright.

DeveloperGuide.md

@@ -11,7 +11,7 @@ This guide focuses on developing for Stirling 2.0, including both the React fron
 **Stirling 2.0** is built using:
 
 **Backend:**
-- Spring Boot (Java 21+, JDK 25 recommended)
+- Spring Boot (requires JDK 25)
 - PDFBox for core PDF operations
 - LibreOffice for document conversions
 - qpdf for PDF optimization
@@ -45,7 +45,7 @@ This guide focuses on developing for Stirling 2.0, including both the React fron
 - [Task](https://taskfile.dev/installation/) — unified command runner (recommended)
 - Docker
 - Git
-- Java JDK 21 or later (JDK 25 recommended)
+- Java JDK 25 (required — JPDFium FFM bindings target JVM 25)
 - Node.js 18+ and npm (required for frontend development)
 - Gradle 7.0 or later (Included within the repo)
 - [uv](https://docs.astral.sh/uv/) — Python package manager (required for engine development)
@@ -61,7 +61,7 @@ This guide focuses on developing for Stirling 2.0, including both the React fron
    cd Stirling-PDF
    ```
 
-2. Install Docker and JDK 21 (or JDK 25 recommended) if not already installed.
+2. Install Docker and JDK 25 if not already installed.
 
 3. Install a recommended Java IDE such as Eclipse, IntelliJ, or VSCode
    1. Only VSCode