Proactively bump golang.org/x/net to v0.12.0

Proactively bump to v0.12.0 to avoid v0.10.0 and v0.11.0, which contain
a regression added by commit
82780d606d.
This commit was later reverted in v0.12.0.

Generated with:

hack/pin-dependency.sh golang.org/x/net v0.12.0 && hack/update-vendor.sh

vendor/golang.org/x/net/html/token.go

@@ -913,7 +913,14 @@ func (z *Tokenizer) readTagAttrKey() {
 		case ' ', '\n', '\r', '\t', '\f', '/':
 			z.pendingAttr[0].end = z.raw.end - 1
 			return
-		case '=', '>':
+		case '=':
+			if z.pendingAttr[0].start+1 == z.raw.end {
+				// WHATWG 13.2.5.32, if we see an equals sign before the attribute name
+				// begins, we treat it as a character in the attribute name and continue.
+				continue
+			}
+			fallthrough
+		case '>':
 			z.raw.end--
 			z.pendingAttr[0].end = z.raw.end
 			return