Document global servlet path matcher configuration

[yashdotdev13]

Fixes #17811

This updates the Spring Security 7 migration guide to explain an alternative way to handle servlet path prefixes in authorization rules.

In addition to the existing explicit basePath("/mvc") example, the guide now shows how to configure a global PathPatternRequestMatcher.Builder bean for applications that use the same servlet path across most authorization rules.

The example uses PathPatternRequestMatcher.withPathPatternParser(...) together with DispatcherServletPath so the configured servlet path is applied consistently when Spring Security builds request matchers from string-based authorization methods.

Validation

• git diff --check
• Reviewed the generated patch locally

Note: the local :spring-security-docs:antora build was blocked by an unrelated Windows esbuild/Gradle environment issue in the JavaScript task, not by this documentation change.