[mod] remove X-XSS-Protection headers

Deprecated header not used by browsers nowadays[1]:

"""In modern browsers, X-XSS-Protection has been deprecated in favor of the
Content-Security-Policy to disable the use of inline JavaScript. Its use can
introduce XSS vulnerabilities in otherwise safe websites. This should not be
used unless you need to support older web browsers that don’t yet support CSP.
It is thus recommended to set the header as X-XSS-Protection: 0."""[2]

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection
[2] https://infosec.mozilla.org/guidelines/web_security#x-xss-protection

Closes: https://github.com/searxng/searxng/issues/3171
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/settings/settings_server.rst

@@ -16,7 +16,6 @@
        image_proxy: false
        default_http_headers:
          X-Content-Type-Options : nosniff
-         X-XSS-Protection : 1; mode=block
          X-Download-Options : noopen
          X-Robots-Tag : noindex, nofollow
          Referrer-Policy : no-referrer

searx/settings.yml

@@ -88,7 +88,6 @@ server:
   method: "POST"
   default_http_headers:
     X-Content-Type-Options: nosniff
-    X-XSS-Protection: 1; mode=block
     X-Download-Options: noopen
     X-Robots-Tag: noindex, nofollow
     Referrer-Policy: no-referrer

tests/unit/settings/user_settings.yml

@@ -19,7 +19,6 @@ server:
   method: "POST"
   default_http_headers:
     X-Content-Type-Options: nosniff
-    X-XSS-Protection: 1; mode=block
     X-Download-Options: noopen
     X-Robots-Tag: noindex, nofollow
     Referrer-Policy: no-referrer