Closes gh-15338

Closes gh-15337

Closes gh-15336

Closes gh-15335

Closes gh-15334

Closes gh-15333

Closes gh-15332

Closes gh-15331

Closes gh-15330

Closes gh-15329

See gh-15328

See gh-15327

Closes gh-15254

Closes gh-15275

Closes gh-15272

Closes gh-15251

Closes gh-15255

Closes gh-14895

Closes gh-15315

Closes gh-15256

Closes gh-15253

Closes gh-15273

Closes gh-15271

Closes gh-15274

Closes gh-15211

This commit adds `ElasticsearchRestHealthIndicator`, a new
`HealthIndicator` for Elasticsearch, using the Elasticsearch "low level
rest client" provided by the
`"org.elasticsearch.client:elasticsearch-rest-client"` dependency.

Note that Spring Boot will auto-configure both low and high level REST
clients, but since the high level one is using the former, a single
health indicator will cover both cases.

See gh-15211