Bump com.apptasticsoftware:rssreader in /data-prepper-plugins/rss-source

Bumps [com.apptasticsoftware:rssreader](https://github.com/w3stling/rssreader) from 3.7.0 to 3.9.3.
- [Release notes](https://github.com/w3stling/rssreader/releases)
- [Commits](w3stling/rssreader@v3.7.0...v3.9.3)

---
updated-dependencies:
- dependency-name: com.apptasticsoftware:rssreader
  dependency-version: 3.9.3
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

data-prepper-plugins/rss-source/build.gradle

@@ -13,7 +13,7 @@ dependencies {
     implementation 'joda-time:joda-time:2.12.7'
     implementation 'com.fasterxml.jackson.core:jackson-core'
     implementation 'com.fasterxml.jackson.core:jackson-databind'
-    implementation 'com.apptasticsoftware:rssreader:3.7.0'
+    implementation 'com.apptasticsoftware:rssreader:3.9.3'
     testImplementation libs.commons.lang3
     testImplementation project(':data-prepper-test-common')
     testImplementation 'org.mock-server:mockserver-junit-jupiter-no-dependencies:5.15.0'