Merge pull request #30009 from Kumataro:cleanupEmscriptionInstallationGuide

js: doc: clean up Emscripten installation guide and remove outdated C++17 note - #30009

### Summary
- Updated `js_setup.markdown` for modern Emscripten (emsdk 6.x) usage.
- Replaced deprecated `./emsdk update` with `git pull`.
- Replaced outdated version recommendations (`2.0.10`) with `latest` and explicit version fallback (`6.0.9`).
- Removed obsolete Note regarding `--cmake_option="-DCMAKE_CXX_STANDARD=17"`, as OpenCV 5 now defaults to C++17.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

doc/js_tutorials/js_setup/js_setup/js_setup.markdown

@@ -16,7 +16,7 @@ To Install Emscripten, follow instructions of [Emscripten SDK](https://emscripte
 
 For example:
 @code{.bash}
-./emsdk update
+git pull # Note: ./emsdk update is deprecated in modern emsdk; use git pull to update the repository.
 ./emsdk install latest
 ./emsdk activate latest
 @endcode
@@ -37,13 +37,14 @@ emcmake sh -c 'echo ${EMSCRIPTEN}'
 @endcode
 
 
-The version 2.0.10 of emscripten is verified for latest WebAssembly. Please check the version of Emscripten to use the newest features of WebAssembly.
+The Emscripten SDK is continuously updated. If you encounter build errors with the latest version, please install and activate a specific version explicitly.
 
 For example:
 @code{.bash}
-./emsdk update
-./emsdk install 2.0.10
-./emsdk activate 2.0.10
+git pull
+./emsdk list # to show available versions. v6.0.9 is used in the following steps.
+./emsdk install 6.0.9
+./emsdk activate 6.0.9
 @endcode
 
 Obtaining OpenCV Source Code
@@ -85,11 +86,6 @@ Building OpenCV.js from Source
 
     @note
     - It requires `python` and `cmake` installed in your development environment.
-    - To build with Emscripten 4.0.20 or later, append --cmake_option="-DCMAKE_CXX_STANDARD=17" .
-      Embind requires C++17 or later since Emscripten 4.0.20.
-      @code{.bash}
-      emcmake python ./opencv/platforms/js/build_js.py build_js --cmake_option="-DCMAKE_CXX_STANDARD=17"
-      @endcode
 
 -#  [Optional] To build the OpenCV.js loader, append `--build_loader`.