Revert "Upgrade to Jetty 9.4.20.v20190813"

This reverts commit f6f99d78 as 9.4.20
includes an incompatible class change where an abstract class in
Jetty's public API was changed to an interface, making it incompatible
with Framework 5.1.9.

See gh-18035

.bomr/bomr.yaml

@@ -16,6 +16,10 @@ bomr:
         versions:
           # Switches to Jarkarta EE API dependencies resulting in duplicate classes
           - '[2.3.2,2.4)'
+      - project: jetty
+        versions:
+          # Contains incompatiable class change
+          - '9.4.20.v20190813'
       - project: saaj-impl
         versions:
           # Switches to Jarkarta EE API dependencies resulting in duplicate classes

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -107,7 +107,7 @@
 		<jedis.version>2.9.3</jedis.version>
 		<jersey.version>2.27</jersey.version>
 		<jest.version>6.3.1</jest.version>
-		<jetty.version>9.4.20.v20190813</jetty.version>
+		<jetty.version>9.4.19.v20190610</jetty.version>
 		<jetty-jsp.version>2.2.0.v201112011158</jetty-jsp.version>
 		<jetty-el.version>8.5.40</jetty-el.version>
 		<jetty-reactive-httpclient.version>1.0.3</jetty-reactive-httpclient.version>