feat(gatling): improve Gatling integration

[CharlesWong]

Improvements to Gatling integration

Addresses the concerns raised in #13390.

Changes

1. Documentation improvements (README.md template)

• Fixed incorrect language reference: "Scala" → "Java" (JHipster 7+ generates Java-based Gatling tests)
• Added microservice-specific note explaining that tests must target the gateway (especially for OAuth2, where microservices are resource-servers only)
• Added instructions for overriding the base URL at runtime via -DbaseURL=...
• Added instructions for controlling load via -Dusers=N -Dramp=S
• Added links to Gatling Maven and Gradle plugin documentation

2. Microservices support fixes (simulation template)

• For microservice applications, the default baseURL now points to the gateway (http://localhost:${gatewayServerPort}) instead of the microservice port. This is the root cause of the authentication failure in Improve Gatling integration #13390 — microservices cannot issue OAuth2 tokens, only the gateway can
• Introduced an apiPrefix variable (/services/<baseName> for microservices, "" for monoliths/gateways) to replace verbose inline conditionals throughout the scenario

3. Simulation quality improvements

• Switched from headerRegex("Location", "(.*)") to header("Location") for capturing the created entity URL (simpler and more correct)
• Added .check(status().is(204)) on DELETE requests to properly assert successful deletion
• Fixed a comment typo: "css or css" → "css or js"
• Improved Javadoc with @code and @pre blocks showing runtime configuration options

Testing

These changes are template-level fixes. The existing snapshot tests cover the default (non-microservice) config and are not affected. Microservice scenarios with OAuth2 will now correctly target the gateway for authentication.

Closes #13390