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