Bump flexmark from 0.61.16 to 0.61.20

Bumps [flexmark](https://github.com/vsch/flexmark-java) from 0.61.16 to 0.61.20.
- [Release notes](https://github.com/vsch/flexmark-java/releases)
- [Commits](vsch/flexmark-java@0.61.16...0.61.20)

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

build.gradle

@@ -196,7 +196,7 @@ dependencies {
         exclude module: "log4j-core"
     }
 
-    implementation 'com.vladsch.flexmark:flexmark:0.61.16'
+    implementation 'com.vladsch.flexmark:flexmark:0.61.20'
     implementation 'com.vladsch.flexmark:flexmark-ext-gfm-strikethrough:0.61.6'
     implementation 'com.vladsch.flexmark:flexmark-ext-gfm-tasklist:0.61.16'