(v3.5.x): Bump org.springframework.boot from 2.6.14 to 2.6.15

Bumps org.springframework.boot from 2.6.14 to 2.6.15.

---
updated-dependencies:
- dependency-name: org.springframework.boot
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

settings.gradle

@@ -16,7 +16,7 @@
 
 pluginManagement {
 	plugins {
-		id "org.springframework.boot" version "2.6.14"
+		id "org.springframework.boot" version "2.6.15"
 		id "io.spring.nohttp" version "0.0.11"
 		id 'org.asciidoctor.jvm.pdf' version '3.3.2'
 		id 'org.asciidoctor.jvm.convert' version '3.3.2'