Revert "Upgrade to Jetty 9.4.20.v20190813"

This reverts commit f6f99d7 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

Author: wilkinsona

.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>