switch maven central badge to shields.io to avoid out-of-sync issues with maven index

Author: stefanseifert

README.md

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 <img src="https://wcm.io/images/favicon-16@2x.png"/> CONGA Plugin for AEM
 ======
 [![Build](https://github.com/wcm-io-devops/conga-aem-plugin/workflows/Build/badge.svg?branch=develop)](https://github.com/wcm-io-devops/conga-aem-plugin/actions?query=workflow%3ABuild+branch%3Adevelop)
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.wcm.devops.conga.plugins/io.wcm.devops.conga.plugins.aem/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.wcm.devops.conga.plugins/io.wcm.devops.conga.plugins.aem)
+[![Maven Central](https://img.shields.io/maven-central/v/io.wcm.devops.conga.plugins/io.wcm.devops.conga.plugins.aem)](https://repo1.maven.org/maven2/io/wcm/devops/conga/plugins/io.wcm.devops.conga.plugins.aem)
 [![Coverage](https://sonarcloud.io/api/project_badges/measure?project=wcm-io-devops_conga-aem-plugin&metric=coverage)](https://sonarcloud.io/summary/new_code?id=wcm-io-devops_conga-aem-plugin)
 
 Documentation: https://devops.wcm.io/conga/plugins/aem/<br/>

src/site/markdown/index.md

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 wcm.io DevOps CONGA Plugin for [Adobe Experience Manager (AEM)][aem].
 
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.wcm.devops.conga.plugins/io.wcm.devops.conga.plugins.aem/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.wcm.devops.conga.plugins/io.wcm.devops.conga.plugins.aem)
+[![Maven Central](https://img.shields.io/maven-central/v/io.wcm.devops.conga.plugins/io.wcm.devops.conga.plugins.aem)](https://repo1.maven.org/maven2/io/wcm/devops/conga/plugins/io.wcm.devops.conga.plugins.aem)
 
 
 ### Documentation