Bump mockito-core from 2.27.0 to 2.28.1

Bumps [mockito-core](https://github.com/mockito/mockito) from 2.27.0 to 2.28.1.
- [Release notes](https://github.com/mockito/mockito/releases)
- [Commits](mockito/mockito@v2.27.0...v2.28.1)

Author: dependabot-preview[bot]

browserup-proxy-core/build.gradle

@@ -98,7 +98,7 @@ dependencies {
         exclude(module: 'netty-handler')
         exclude(module: 'netty-transport')
     }
-    testImplementation 'org.mockito:mockito-core:2.27.0'
+    testImplementation 'org.mockito:mockito-core:2.28.1'
     testImplementation 'org.seleniumhq.selenium:selenium-firefox-driver:3.141.59'
     testImplementation 'org.apache.httpcomponents:httpclient:4.5.8'
 }

browserup-proxy-mitm/build.gradle

@@ -80,5 +80,5 @@ dependencies {
     testImplementation 'org.codehaus.groovy:groovy-all:2.5.7'
     testImplementation 'org.hamcrest:hamcrest:2.1'
     testImplementation 'org.hamcrest:hamcrest-library:2.1'
-    testImplementation 'org.mockito:mockito-core:2.27.0'
+    testImplementation 'org.mockito:mockito-core:2.28.1'
 }

browserup-proxy-rest/build.gradle

@@ -82,7 +82,7 @@ dependencies {
     testImplementation 'org.hamcrest:hamcrest:2.1'
     testImplementation 'org.hamcrest:hamcrest-library:2.1'
     testImplementation 'org.mock-server:mockserver-netty:5.5.4'
-    testImplementation 'org.mockito:mockito-core:2.27.0'
+    testImplementation 'org.mockito:mockito-core:2.28.1'
     testImplementation 'org.seleniumhq.selenium:selenium-api:3.4.0'
     testImplementation 'org.awaitility:awaitility:3.1.6'
     testImplementation 'xyz.rogfam:littleproxy:2.0.0-beta-3'