Add dependency on BouncyCastle for JDK 15+

With a recent upgrade to an early access build for JDK 15,
ServerHttpsRequestIntegrationTests began failing since Netty's
SelfSignedCertificate could not be properly initialized.

This commit adds a test fixture dependency on BouncyCastle which is now
needed by Netty's SelfSignedCertificate on JDK 15 or higher, since JDK 15
no longer supports the javax.security.cert.X509Certificate.

See: https://bugs.openjdk.java.net/browse/JDK-8241039

build.gradle

@@ -207,6 +207,7 @@ configure(allprojects) { project ->
 			}
 			dependency "org.skyscreamer:jsonassert:1.5.0"
 			dependency "com.jayway.jsonpath:json-path:2.4.0"
+			dependency "org.bouncycastle:bcpkix-jdk15on:1.65"
 
 			dependencySet(group: 'org.apache.tiles', version: '3.0.8') {
 				entry 'tiles-api'

spring-web/spring-web.gradle

@@ -83,4 +83,7 @@ dependencies {
 	testFixturesImplementation("io.projectreactor:reactor-test")
 	testFixturesImplementation("org.apache.taglibs:taglibs-standard-jstlel")
 	testFixturesImplementation("org.assertj:assertj-core")
+	testFixturesImplementation("org.bouncycastle:bcpkix-jdk15on") {
+		because("needed by Netty's SelfSignedCertificate on JDK 15+")
+	}
 }