Remove jcenter dependencies from build (#1436)

Motivation:
jcenter is deprecating bintray, and it will soon stop serving artifacts
eventually.

Modifications:
- Remove all references to jcenter from the build

Result:
No more direct references to jcenter from the build avoids dependency on
soon to be decommissioned repository.

README.adoc

@@ -15,18 +15,6 @@ See the link:https://docs.servicetalk.io/[ServiceTalk docs] for more information
 
 ServiceTalk releases are available on link:https://repo1.maven.org/maven2/io/servicetalk/[Maven Central].
 
-For Gradle as well as other build tools that don't use Maven Central as a default repository, additional configuration
-is required.
-
-.Gradle, build.gradle
-[source,groovy]
-----
-repositories {
-  jcenter() // combines Maven Central and other popular repositories
-}
-----
-
-
 Refer to the link:https://docs.servicetalk.io/[ServiceTalk docs] for various examples that will get you started with the
 different features of ServiceTalk.
 
@@ -58,35 +46,6 @@ the project. Setting the `JAVA_TOOL_OPTIONS` env var to `-Dfile.encoding=UTF-8`
 non-UTF-8 environments. Editors and IDEs must also support UTF-8 in order to successfully edit ServiceTalk's source
 code.
 
-==== Gradle Repositories
-
-ServiceTalk's build produces custom Gradle plugins and thus has regular (i.e. non-`buildscript`) dependencies
-on other plugins. This is the reason why the repositories that are provided if none are configured globally are the
-following:
-
-[source,groovy]
-----
-allprojects {
-  buildscript {
-    repositories {
-      jcenter()
-      maven { url "https://plugins.gradle.org/m2/" }
-    }
-  }
-  repositories {
-    jcenter()
-    maven { url "https://plugins.gradle.org/m2/" }
-  }
-}
-----
-
-If you have defined repositories or repository mirrors in your global Gradle config (`~/.gradle/init.gradle`),
-the build will detect them and attempt to inherit `buildscript` repositories into the main `repositories`
-of the sub-projects that produce custom Gradle plugins.
-
-NOTE: This inheritance mechanism can be disabled by setting a Gradle property: +
-      `-PdisableInheritBuildscriptRepositories`.
-
 ==== Build Commands
 
 You should be able to run the following command to build ServiceTalk and verify that all

build.gradle

@@ -18,12 +18,10 @@ if (!repositories) {
   allprojects {
     buildscript {
       repositories {
-        jcenter()
         maven { url "https://plugins.gradle.org/m2/" }
       }
     }
     repositories {
-      jcenter()
       maven { url "https://plugins.gradle.org/m2/" }
     }
   }

buildSrc/build.gradle

@@ -16,7 +16,6 @@
 
 if (!repositories) {
   repositories {
-    jcenter()
     maven { url "https://plugins.gradle.org/m2/" }
   }
 } else {

docs/generation/build.gradle

@@ -17,7 +17,6 @@
 buildscript {
   if (!repositories) {
     repositories {
-      jcenter()
       maven { url "https://plugins.gradle.org/m2/" }
     }
   }

docs/generation/gradle/validateSite.gradle

@@ -16,7 +16,7 @@
 
 if (!repositories) {
   repositories {
-    jcenter()
+    mavenCentral()
   }
 }