Bump org.bouncycastle:bcpkix-jdk15on from 1.66 to 1.70 (#131)

Bumps [org.bouncycastle:bcpkix-jdk15on](https://github.com/bcgit/bc-java) from 1.66 to 1.70.
- [Release notes](https://github.com/bcgit/bc-java/releases)
- [Changelog](https://github.com/bcgit/bc-java/blob/master/docs/releasenotes.html)
- [Commits](https://github.com/bcgit/bc-java/commits)

---
updated-dependencies:
- dependency-name: org.bouncycastle:bcpkix-jdk15on
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

java-spiffe-core/build.gradle

@@ -94,7 +94,7 @@ dependencies {
     testFixturesImplementation group: 'com.nimbusds', name: 'nimbus-jose-jwt', version: "${nimbusVersion}"
 
     // using bouncy castle for generating X.509 certs for testing purposes
-    testFixturesImplementation group: 'org.bouncycastle', name: 'bcpkix-jdk15on', version: '1.66'
+    testFixturesImplementation group: 'org.bouncycastle', name: 'bcpkix-jdk15on', version: '1.70'
     testFixturesImplementation group: 'org.apache.commons', name: 'commons-lang3', version: '3.12.0'
 }