Add Maven Central repo to pom.xml (#273)

## Checklist before submitting

- [X] Did you read the [contributing
guide](https://github.com/G-Research/spark-dgraph-connector/blob/contributing-guidelines/CONTRIBUTING.md)?
- [ ] Did you update the docs?
- [ ] Did you write any tests to validate this change?  

## Description

Adds Maven Central repo to speed up dependency download and CI.

## Review process for approval

1. All tests and other checks must succeed.
2. At least one core contributors must review and approve.
3. If a core contributor requests changes, they must be addressed.

pom.xml

@@ -45,23 +45,37 @@
   </properties>
 
   <repositories>
-    <!-- required to resolve GraphFrames dependency -->
+    <!-- the official Maven Central repository -->
     <repository>
-      <id>spark packages</id>
-      <name>Spark Packages</name>
-      <url>https://repos.spark-packages.org</url>
+      <id>maven-central</id>
+      <name>Maven Central</name>
+      <url>https://repo1.maven.org/maven2</url>
+      <releases><enabled>true</enabled></releases>
+      <snapshots><enabled>false</enabled></snapshots>
     </repository>
     <!-- the official source for Spark releases, mirrored to Maven central -->
     <repository>
-      <id>apache releases</id>
+      <id>apache-releases</id>
       <name>Apache Releases</name>
       <url>https://repository.apache.org/content/repositories/releases/</url>
+      <releases><enabled>true</enabled></releases>
+      <snapshots><enabled>false</enabled></snapshots>
     </repository>
     <!-- required to resolve Spark SNAPSHOT dependencies -->
     <repository>
-      <id>apache snapshots</id>
+      <id>apache-snapshots</id>
       <name>Apache Snapshots</name>
       <url>https://repository.apache.org/snapshots</url>
+      <releases><enabled>false</enabled></releases>
+      <snapshots><enabled>true</enabled></snapshots>
+    </repository>
+    <!-- required to resolve GraphFrames dependency -->
+    <repository>
+      <id>spark-packages</id>
+      <name>Spark Packages</name>
+      <url>https://repos.spark-packages.org</url>
+      <releases><enabled>true</enabled></releases>
+      <snapshots><enabled>false</enabled></snapshots>
     </repository>
   </repositories>