Bump org.springframework:spring-framework-bom from 6.1.20 to 6.1.21

---
updated-dependencies:
- dependency-name: org.springframework:spring-framework-bom
  dependency-version: 6.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

gradle/libs.versions.toml

@@ -13,7 +13,7 @@ org-jetbrains-kotlin = "1.9.25"
 org-jetbrains-kotlinx = "1.8.1"
 org-mockito = "5.11.0"
 org-opensaml = "4.3.2"
-org-springframework = "6.1.20"
+org-springframework = "6.1.21"
 
 [libraries]
 ch-qos-logback-logback-classic = "ch.qos.logback:logback-classic:1.5.18"