[MODEL] Update README. `7.x` is supported.

[sensuikan1973]

Although https://github.com/elastic/elasticsearch-rails/tree/main/elasticsearch-model#compatibility says that 7.x tag is not supported, 7.x has been supported by https://github.com/elastic/elasticsearch-rails/commit/a0f14d96fab54b64cb3b8cbacd6476aba2dfa78d . Then, I update README.

ref

• https://github.com/elastic/elasticsearch-rails/tree/main/elasticsearch-persistence#compatibility
• https://github.com/elastic/elasticsearch-rails/tree/main/elasticsearch-rails#compatibility

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