Update docker.elastic.co/elasticsearch/elasticsearch Docker tag to v9.1.5 (elastic#136146)

This PR contains the following updates:

| Package | Update | Change | |---|---|---| |
[docker.elastic.co/elasticsearch/elasticsearch](https://www.elastic.co/products/elasticsearch)
([source](https://redirect.github.com/elastic/elasticsearch)) | patch |
`9.1.3` -> `9.1.5` |

---

### Release Notes

<details> <summary>elastic/elasticsearch
(docker.elastic.co/elasticsearch/elasticsearch)</summary>

###
[`v9.1.5`](https://redirect.github.com/elastic/elasticsearch/releases/tag/v9.1.5):
Elasticsearch 9.1.5

[Compare
Source](https://redirect.github.com/elastic/elasticsearch/compare/v9.1.4...v9.1.5)

Downloads: https://elastic.co/downloads/elasticsearch Release notes:
https://www.elastic.co/docs/release-notes/elasticsearch#elasticsearch-9.1.5-release-notes

###
[`v9.1.4`](https://redirect.github.com/elastic/elasticsearch/releases/tag/v9.1.4):
Elasticsearch 9.1.4

[Compare
Source](https://redirect.github.com/elastic/elasticsearch/compare/v9.1.3...v9.1.4)

Downloads: https://elastic.co/downloads/elasticsearch Release notes:
https://www.elastic.co/docs/release-notes/elasticsearch#elasticsearch-9.1.4-release-notes

</details>

---

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This PR has been generated by [Renovate
Bot](https://redirect.github.com/renovatebot/renovate).
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Author: elastic-renovate-prod[bot]

x-pack/qa/repository-old-versions-compatibility/src/javaRestTest/resources/README.md

@@ -35,10 +35,10 @@ docker run -d --name es \
 -v ${SHARED_FOLDER}/data:/usr/share/elasticsearch/data \
 -v ${SHARED_FOLDER}/snapshots:/usr/share/elasticsearch/snapshots \
 --env "discovery.type=single-node" \
-docker.elastic.co/elasticsearch/elasticsearch:9.1.3
+docker.elastic.co/elasticsearch/elasticsearch:9.1.5
 
 // Version 6
-docker.elastic.co/elasticsearch/elasticsearch:9.1.3
+docker.elastic.co/elasticsearch/elasticsearch:9.1.5
 ```
 
 ### Create Index Version 5