[SearchPipelines] Lower compatibility version to 2.7

[msfroh]

Description

The search pipelines code should be available starting in OpenSearch 2.7, so compatibility checks should start from that version.

Note that the Gradle checks for this PR may fail until #7075 is merged.

Issues Resolved

Follow-on from #7075, which is a backport of #6587, which resolves opensearch-project/search-processor#97.

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/13971/
• CommitID: 4974548
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/13973/
• CommitID: 3a4b3fb
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[reta]

Gradle Check (Jenkins) Run Completed with:

* **RESULT:** FAILURE ❌

* **URL:** https://build.ci.opensearch.org/job/gradle-check/13973/

* **CommitID:** [3a4b3fb](https://github.com/opensearch-project/OpenSearch/commit/3a4b3fbe7214158d51c03f149ec6d86af2d7a88f)
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure [a flaky test](https://github.com/opensearch-project/OpenSearch/blob/main/DEVELOPER_GUIDE.md#flaky-tests) unrelated to your change?

@msfroh the BWC test failures are OK at this moment, once the pull request to 2.x gets in, the tests will be green, thank you.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/14006/
• CommitID: 3a4b3fb