Hide `@loopback/openapi-v3` from docs, examples and CLI templates

[bajtos]

In a follow-up to #5659 and #5550, we should update our docs, examples and CLI templates to load openapi-v3 APIs from @loopback/rest, so that framework users do not need to learn about @loopback/openapi-v3 package.

Acceptance criteria

In the search commands listed below, rg refers to RipGrep.

• Remove @loopback/openapi-v3 from all package.json files EXCEPT packages/rest/package.json. Update package-lock.json files where necessary. Search command:

  $ rg loopback/openapi-v3 -g package.json

  --> feat: remove openapi-v3 from apiconnect dependencies #5927, refactor: remove @loopback/openapi-v3 from dependencies #5932

• Remove @loopback/openapi-v3 in CLI templates and update tests as necessary.

  • packages/cli/generators/project/templates/package.json.ejs
  • packages/cli/generators/project/templates/package.plain.json.ejs.

  --> feat(cli): remove openapi-v3 from project template dependencies #5926

• Update code in examples, replace imports from @loopback/openapi-v3 to import from @loopback/rest instead. Search command:

  $ rg loopback/openapi-v3 examples/ -g '!package.json'

  --> feat: remove openapi-v3 from apiconnect dependencies #5927, refactor: remove @loopback/openapi-v3 from dependencies #5932

• Update code snippets in our documentation, replace imports from @loopback/openapi-v3 to import from @loopback/rest instead. Search command:

  rg loopback/openapi-v3 docs/site

  --> docs: remove references to openapi-v3 #5933

• Final check: review remaining references to @loopback/openapi-v3 and verify they are all desired. Search command:

  $ rg loopback/openapi-v3 -g '!CHANGELOG.md'