About This HTML Issue
The xmlns:inkscape attribute declares the namespace Inkscape uses for its own editor data, and HTML doesn't allow prefixed namespace declarations on <svg> or any other element.
Inkscape's default file format, Inkscape SVG, puts xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape" on the root <svg> element. The prefix marks attributes that only Inkscape reads: inkscape:version records the release that saved the file, and inkscape:groupmode="layer" with an inkscape:label turns a <g> into a named layer. Inkscape 0.92 also writes inkscape:connector-curvature="0" on paths, and inkscape:export-filename remembers where the last PNG export went. Browsers ignore all of them, so the image looks the same without them.
In an HTML page the declaration does nothing. The HTML parser doesn't handle namespace prefixes, so xmlns:inkscape is a plain attribute with a colon in its name, and the validator reports it on any element that carries it. Each inkscape: attribute gets its own message as well, such as Attribute "inkscape:version" not allowed on element "svg" at this point. Inkscape SVG files usually declare xmlns:sodipodi next to it, for attributes like sodipodi:docname, and the validator reports that declaration separately.
Delete xmlns:inkscape and every attribute that starts with inkscape:. Removing only the declaration leaves the attribute errors behind. The layers turn into plain groups and lose their labels, so if a stylesheet or script needs to find one, use the group's id (Inkscape assigns ids like layer1) or add a class.
When you clean up a standalone .svg file, remove the declaration and the attributes together. Delete xmlns:inkscape but leave a single inkscape:label behind, and the file stops being well-formed XML, because it uses a prefix it never declares. Browsers then refuse to display it (Firefox reports an "unbound prefix").
To get a copy without the Inkscape markup, use File > Save a Copy and choose Optimized SVG or Plain SVG. Optimized SVG runs Scour, which strips editor data unless you check its "Keep editor data" option. Plain SVG drops the inkscape: and sodipodi: markup but still writes xmlns:svg, which the validator also rejects.
Keep the Inkscape SVG as the file you edit. A copy without the editor data reopens without its layers, and shapes drawn with the star tool come back as plain paths. If a build step inlines your SVGs, SVGO removes the same markup with removeEditorsNSData, which is part of its default preset.
Invalid example
<svg xmlns="http://www.w3.org/2000/svg"
xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
inkscape:version="1.3.2"
width="24" height="24" viewBox="0 0 24 24">
<g inkscape:label="Icon" inkscape:groupmode="layer" id="layer1">
<path d="M5 12l5 5L20 7" fill="none" stroke="currentColor" stroke-width="2" />
</g>
</svg>
Valid example
<svg xmlns="http://www.w3.org/2000/svg"
width="24" height="24" viewBox="0 0 24 24">
<g id="layer1">
<path d="M5 12l5 5L20 7" fill="none" stroke="currentColor" stroke-width="2" />
</g>
</svg>
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