fix: Use the correct deprecated syntax in code

[shogo82148]

The godoc of deprecated types, values and functions should be match the regexp (^|\n\s*\n)\s*Deprecated:.

https://github.com/golang/pkgsite/blob/8996ff632abee854aef1b764ca0501f262f8f523/internal/godoc/dochtml/deprecated.go#L12-L13

pkg.go.dev marks these items deprecated and show them using a special style.

e.g. https://pkg.go.dev/net/http#CloseNotifier

[lahirumaramba]

The base branch needs to be changed to dev. Any chance you could fix that in your shogo82148:mark-deprecated-on-pkgsite branch?