Convert max file name length to 255 (go-gitea#30489)

Quick/Partly fix go-gitea#29907

In Linux and MacOS, by default the max file name length is 255.
In windows, it depends on the version and settings, and has no file name
length limitation, but has path length limitation.
By default it is 260, considering path length is longer than filename,
so I think it is ok to do this.

For Windows, see
https://learn.microsoft.com/windows/win32/fileio/maximum-file-path-limitation?tabs=registry
For Linux, see
https://github.com/torvalds/linux/blob/master/include/uapi/linux/limits.h#L12-L13
For MacOS, see
https://discussions.apple.com/thread/254788848?sortBy=best

Author: yp05327

templates/repo/editor/edit.tmpl

@@ -15,7 +15,7 @@
 					{{range $i, $v := .TreeNames}}
 						<div class="breadcrumb-divider">/</div>
 						{{if eq $i $l}}
-							<input id="file-name" maxlength="500" value="{{$v}}" placeholder="{{ctx.Locale.Tr "repo.editor.name_your_file"}}" data-editorconfig="{{$.EditorconfigJson}}" required autofocus>
+							<input id="file-name" maxlength="255" value="{{$v}}" placeholder="{{ctx.Locale.Tr "repo.editor.name_your_file"}}" data-editorconfig="{{$.EditorconfigJson}}" required autofocus>
 							<span data-tooltip-content="{{ctx.Locale.Tr "repo.editor.filename_help"}}">{{svg "octicon-info"}}</span>
 						{{else}}
 							<span class="section"><a href="{{$.BranchLink}}/{{index $.TreePaths $i | PathEscapeSegments}}">{{$v}}</a></span>

templates/repo/editor/upload.tmpl

@@ -13,7 +13,7 @@
 					{{range $i, $v := .TreeNames}}
 						<div class="breadcrumb-divider">/</div>
 						{{if eq $i $l}}
-							<input type="text" id="file-name" maxlength="500" value="{{$v}}" placeholder="{{ctx.Locale.Tr "repo.editor.add_subdir"}}" autofocus>
+							<input type="text" id="file-name" maxlength="255" value="{{$v}}" placeholder="{{ctx.Locale.Tr "repo.editor.add_subdir"}}" autofocus>
 							<span data-tooltip-content="{{ctx.Locale.Tr "repo.editor.filename_help"}}">{{svg "octicon-info"}}</span>
 						{{else}}
 							<span class="section"><a href="{{$.BranchLink}}/{{index $.TreePaths $i | PathEscapeSegments}}">{{$v}}</a></span>