Use spring-boot-starter-parent in spring-batch-elasticsearch

[scordio]

Similar to #159, #163, and #165, this PR:

• Uses the spring-boot-starter-parent to keep all the dependency and plugin versions aligned
• Uses the flatten-maven-plugin with ossrh flatten mode to strip all the unnecessary details from the final POM

Flattened POM

<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.springframework.batch.extensions</groupId>
  <artifactId>spring-batch-elasticsearch</artifactId>
  <version>0.1.0-SNAPSHOT</version>
  <name>Spring Batch Elasticsearch</name>
  <description>Spring Batch extension for Elasticsearch</description>
  <url>https://github.com/spring-projects/spring-batch-extensions/tree/main/spring-batch-elasticsearch</url>
  <licenses>
    <license>
      <name>Apache-2.0</name>
      <url>https://www.apache.org/licenses/LICENSE-2.0.txt</url>
      <distribution>repo</distribution>
    </license>
  </licenses>
  <scm>
    <url>https://github.com/spring-projects/spring-batch-extensions/tree/main/spring-batch-elasticsearch</url>
  </scm>
  <dependencies>
    <dependency>
      <groupId>org.springframework.batch</groupId>
      <artifactId>spring-batch-core</artifactId>
      <version>3.0.10.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.data</groupId>
      <artifactId>spring-data-elasticsearch</artifactId>
      <version>2.1.23.RELEASE</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>
</project>

[scordio]

This is the latest 1.5 available.

Moving to a higher major version requires refactoring of the item reader and writer due to breaking changes, which would be beyond the scope of this PR.

[scordio]

Hi @fmbenhassine, I understand from #166 (comment) that @hpoettker will maintain this module.

Since I couldn't add their handle for review, I assume the onboarding is not complete yet.

Therefore, I haven't added any developers section to the POM and added this parameter to prevent pulling the ones defined in the spring-boot-starter-parent.