MINOR: Update jacoco to 0.8.7 for JDK 16 support (#10654)

Details:
* https://github.com/jacoco/jacoco/releases/tag/v0.8.6
* https://github.com/jacoco/jacoco/releases/tag/v0.8.7

Ran `./gradlew clients:reportCoverage -PenableTestCoverage=true -Dorg.gradle.parallel=false`
successfully with Java 15 (see gradle/gradle#15730 and
scoverage/gradle-scoverage#150 for the reason why 
`-Dorg.gradle.parallel=false` is required).

Also updated `README.md` to include `-Dorg.gradle.parallel=false` alongside `reportCoverage`.

Reviewers: Chia-Ping Tsai <chia7712@gmail.com>

README.md

@@ -60,11 +60,11 @@ See [Test Retry Gradle Plugin](https://github.com/gradle/test-retry-gradle-plugi
 ### Generating test coverage reports ###
 Generate coverage reports for the whole project:
 
-    ./gradlew reportCoverage -PenableTestCoverage=true
+    ./gradlew reportCoverage -PenableTestCoverage=true -Dorg.gradle.parallel=false
 
 Generate coverage for a single module, i.e.: 
 
-    ./gradlew clients:reportCoverage -PenableTestCoverage=true
+    ./gradlew clients:reportCoverage -PenableTestCoverage=true -Dorg.gradle.parallel=false
 
 ### Building a binary release gzipped tar ball ###
     ./gradlew clean releaseTarGz

gradle/dependencies.gradle

@@ -67,7 +67,7 @@ versions += [
   easymock: "4.3",
   jackson: "2.10.5",
   jacksonDatabind: "2.10.5.1",
-  jacoco: "0.8.5",
+  jacoco: "0.8.7",
   javassist: "3.27.0-GA",
   jetty: "9.4.39.v20210325",
   jersey: "2.34",