chore(deps): update dependency sphinx to v8 (release/5.0.x) (#4546)

* chore(deps): update dependency sphinx to v8

* Replace removed `style` variable in HTML templates

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Jochen Schalanda <jochen@schalanda.name>

Author: renovate[bot]

docs/requirements.txt

@@ -1 +1 @@
-sphinx<7
+sphinx<9

docs/source/_themes/metrics/layout.html

@@ -19,7 +19,9 @@
     {%- for cssfile in css_files %}
     <link rel="stylesheet" href="{{ pathto(cssfile, 1) }}" type="text/css"/>
     {%- endfor %}
+    {%- for style in styles -%}
     <link rel="stylesheet" href="{{ pathto('_static/' + style, 1) }}" type="text/css"/>
+    {%- endfor -%}
     {%- if favicon %}
     <link rel="shortcut icon" href="{{ pathto('_static/' + favicon, 1) }}"/>
     {%- endif %}