fix: normalize backslash in getStaticPaths() for windows (#53876)

This normalizes the backslash for windows users who accidentally use `\` in the request paths instead of the expected `/`.

For example, in nodejs/nodejs.org#5617 the slash was hardcoded for split that caused the backlash to remain the the `getStaticPaths()` result.

- Fixes #51225

packages/next/src/build/utils.ts

@@ -1035,6 +1035,7 @@ export async function buildStaticPaths({
                   .join('/')
               : escapePathDelimiters(paramValue as string, true)
           )
+          .replace(/\\/g, '/')
           .replace(/(?!^)\/$/, '')
 
         encodedBuiltPage = encodedBuiltPage
@@ -1044,6 +1045,7 @@ export async function buildStaticPaths({
               ? (paramValue as string[]).map(encodeURIComponent).join('/')
               : encodeURIComponent(paramValue as string)
           )
+          .replace(/\\/g, '/')
           .replace(/(?!^)\/$/, '')
       })