fix: drop response.charset because charset deprecated (#206)

I took a quick peek at upstream to see if this has any backwards-breaking issues whereby we'd need to check the `werkzeug` version, but from my reading it looks it should be fine most of the time.

Also, this was deprecated back in April's `2.3.0` release and then removed in the recent `3.0.0` release, so I suspect most of our userbase will have already migrated to those versions.

Given this lib is a dev-tooling library not typically used in production, I'm not too worried about ensuring we support the 0.01% case where someone is using an old version of `werkzeug` + a custom charset.

More context:
* pallets/werkzeug#2602
* pallets/werkzeug#2641
* pallets/werkzeug#2768

src/flask_debugtoolbar/__init__.py

@@ -231,9 +231,9 @@ def process_response(self, response):
             return response
 
         if 'gzip' in response.headers.get('Content-Encoding', ''):
-            response_html = gzip_decompress(response.data).decode(response.charset)
+            response_html = gzip_decompress(response.data).decode()
         else:
-            response_html = response.data.decode(response.charset)
+            response_html = response.get_data(as_text=True)
 
         no_case = response_html.lower()
         body_end = no_case.rfind('</body>')