Upgrade to JDK 11

We currently advertise that we support "JDK 8". However, the public
updates of Java SE 8 for personal users will end soon [1].

JDK 11 is the next Long-Term-Support (LTS) release after JDK 8 [1].
It is better for us to keep updated with the latest release of JDK.

Let's update our target JDK to version 11, with the following steps:

  * We use openjfx-monocle version jdk-11+26 since that is the latest
    version of openjfx-monocle that supports JDK 11 [2].

  * We bump the target and source compatibility of Gradle to JDK11.

  * We update Travis and AppVeyor configs to use JDK11 as runtime
    environment.

  * We remove the add-on in Travis config because it is redundant for
    JDK 11 [3].

  * We make it clear in the User Guide / Developer Guide that we only
    support JDK 11 and above (not JDK 8, 9, 10).

[1] https://www.oracle.com/technetwork/java/java-se-support-roadmap.html
[2] https://github.com/TestFX/Monocle
[3] https://docs.travis-ci.com/user/languages/java/#using-java-10-and-later

.travis.yml

@@ -1,7 +1,7 @@
 language: java
 matrix:
   include:
-    - jdk: oraclejdk8
+    - jdk: oraclejdk11
 
 script: >-
     ./config/travis/run-checks.sh &&
@@ -14,11 +14,6 @@ deploy:
   on:
     branch: master
 
-addons:
-  apt:
-    packages:
-      - oracle-java8-installer
-
 before_cache:
   - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
   - rm -fr $HOME/.gradle/caches/*/plugin-resolution/

appveyor.yml

@@ -11,7 +11,7 @@ test_script:
     - gradlew.bat --no-daemon headless allTests
 
 environment:
-    JAVA_HOME: C:\Program Files\Java\jdk1.8.0  # Use 64-bit Java
+    JAVA_HOME: C:\Program Files\Java\jdk11  # Use 64-bit Java
 
 # Files/folders to preserve between builds to speed them up
 cache:

build.gradle

@@ -17,8 +17,8 @@ plugins {
 // Specifies the entry point of the application
 mainClassName = 'seedu.address.Main'
 
-sourceCompatibility = JavaVersion.VERSION_1_8
-targetCompatibility = JavaVersion.VERSION_1_8
+sourceCompatibility = JavaVersion.VERSION_11
+targetCompatibility = JavaVersion.VERSION_11
 
 repositories {
     mavenCentral()
@@ -72,7 +72,7 @@ dependencies {
 
     testImplementation group: 'org.junit.jupiter', name: 'junit-jupiter-api', version: jUnitVersion
 
-    testRuntimeOnly group: 'org.testfx', name: 'openjfx-monocle', version: '8u76-b04'
+    testRuntimeOnly group: 'org.testfx', name: 'openjfx-monocle', version: 'jdk-11+26'
     testRuntimeOnly group: 'org.junit.jupiter', name: 'junit-jupiter-engine', version: jUnitVersion
 }
 

docs/DeveloperGuide.adoc

@@ -20,12 +20,7 @@ By: `Team SE-EDU`      Since: `Jun 2016`      Licence: `MIT`
 
 === Prerequisites
 
-. *JDK `8`* (revision `1.8.0_201` or later)
-+
-[NOTE]
-Only JDK 8 is supported. +
-This app will not work with later major JDK releases such as JDK 9, 10, 11, etc.
-+
+. *JDK `11`* or above
 . *IntelliJ* IDE
 +
 [NOTE]
@@ -881,7 +876,7 @@ _{More to be added}_
 [appendix]
 == Non Functional Requirements
 
-.  Should work on any <<mainstream-os,mainstream OS>> as long as it has Java `8` (revision `1.8.0_201` or higher) installed.
+.  Should work on any <<mainstream-os,mainstream OS>> as long as it has Java `11` or above installed.
 .  Should be able to hold up to 1000 persons without a noticeable sluggishness in performance for typical usage.
 .  A user with above average typing speed for regular English text (i.e. not code, not system admin commands) should be able to accomplish most of the tasks faster using commands than using the mouse.
 

docs/UserGuide.adoc

@@ -22,12 +22,7 @@ AddressBook Level 4 (AB4) is for those who *prefer to use a desktop app for mana
 
 == Quick Start
 
-.  Ensure you have Java `8` (revision `1.8.0_201` or later) installed in your Computer.
-+
-[NOTE]
-Only Java 8 is supported. +
-This app will not work with later major Java releases such as Java 9, 10, 11, etc.
-+
+.  Ensure you have Java `11` or above installed in your Computer.
 .  Download the latest `addressbook.jar` link:{repoURL}/releases[here].
 .  Copy the file to the folder you want to use as the home folder for your Address Book.
 .  Double-click the file to start the app. The GUI should appear in a few seconds.