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]

Hi @hpoettker, I just added you to this PR but feel free to close it if you plan some more aggressive refactoring 🙂

[fmbenhassine]

@scordio Thank you for this PR and for following up on this!

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

The onboarding was completed. @hpoettker is part of this org now and has push access to the repository.

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. feel free to close it if you plan some more aggressive refactoring 🙂

Since Batch 6 and Boot 4 are out, and in order to reduce the maintenance effort of multiple versions, I would make v1 of this elasticsearch module based on the latest versions (as Spring Batch 5.2 will be out of OSS support in a few months from now). But I will let @hpoettker decide what to do with this regard.