Add [other].SHOW_FOOTER_POWERED_BY setting to hide Powered by (#30253)

This allows you to hide the "Powered by" text in footer via
`SHOW_FOOTER_POWERED_BY` flag in configuration.

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Co-authored-by: silverwind <me@silverwind.io>

docs/content/administration/config-cheat-sheet.en-us.md

@@ -1429,5 +1429,6 @@ Like `uses: https://gitea.com/actions/checkout@v4` or `uses: http://your-git-ser
 
 - `SHOW_FOOTER_VERSION`: **true**: Show Gitea and Go version information in the footer.
 - `SHOW_FOOTER_TEMPLATE_LOAD_TIME`: **true**: Show time of template execution in the footer.
+- `SHOW_FOOTER_POWERED_BY`: **true**: Show the "powered by" text in the footer.
 - `ENABLE_SITEMAP`: **true**: Generate sitemap.
 - `ENABLE_FEED`: **true**: Enable/Disable RSS/Atom feed.

docs/content/administration/config-cheat-sheet.zh-cn.md

@@ -1353,5 +1353,6 @@ PROXY_HOSTS = *.github.com
 
 - `SHOW_FOOTER_VERSION`: **true**: 在页面底部显示Gitea的版本。
 - `SHOW_FOOTER_TEMPLATE_LOAD_TIME`: **true**: 在页脚显示模板执行的时间。
+- `SHOW_FOOTER_POWERED_BY`: **true**: 在页脚显示“由...提供动力”的文本。
 - `ENABLE_SITEMAP`: **true**: 生成sitemap.
 - `ENABLE_FEED`: **true**: 是否启用RSS/Atom