Remove missing `sizes` attribute error when using `loading="lazy"`—swap to use `sizes="auto"` instead.

[zachleat]

Note that "auto" is already supported as a value for sizes in this plugin. This issue is specifically for the error message at:

eleventy-img/generate-html.js

Line 90 in e17ff95

let missingSizesErrorMessage = `Missing \`sizes\` attribute on eleventy-img shortcode from: ${originalSrc || attributes.src}`;

IMPORTANTLY sizes="auto" is not yet supported anywhere.

https://groups.google.com/a/chromium.org/g/blink-dev/c/OAsmCbjPJz0/m/jzuTJzs1AAAJ

[zachleat]

Shipping with v6.0.0-beta.1. Beware the browser support here: https://caniuse.com/mdn-html_elements_img_sizes_auto

We’re only using sizes="auto" when sizes is not supplied in markup and only to avoid a build-time error when also using loading="lazy"