fix(updater): format Update.date to RFC 3339 (#2573)

* fix(updater): format `Update.date` to RFC 3339

* Messed up on argument in #2572

* Format

* Update example

* Avoid extra to_string

* Deprecate `Update.available`

plugins/updater/src/error.rs

@@ -77,6 +77,8 @@ pub enum Error {
     InvalidHeaderValue(#[from] http::header::InvalidHeaderValue),
     #[error(transparent)]
     InvalidHeaderName(#[from] http::header::InvalidHeaderName),
+    #[error("Failed to format date")]
+    FormatDate,
     /// The configured updater endpoint must use a secure protocol like `https`
     #[error("The configured updater endpoint must use a secure protocol like `https`.")]
     InsecureTransportProtocol,