deps: update dependency com.google.cloud:google-cloud-pubsublite-bom to v1.15.20 (#7306)

Author: renovate-bot

google-cloud-bom/pom.xml

@@ -231,7 +231,7 @@
  <dependency>
  <groupId>com.google.cloud</groupId>
  <artifactId>google-cloud-pubsublite-bom</artifactId>
- <version>1.15.19</version>
+ <version>1.15.20</version>
  <type>pom</type>
  <scope>import</scope>
  </dependency>