improvement: maintenance+cleanup+fix (refraction-networking#252)

* ci: use latest Go 1.21 build

Use `1.21.x` instead of `1.21.0` to automatically select the latest.

* fix: remove unused fipsonly package

Remove an unused package that was unintendedly introduced as a conditional dependency of upstream

* update: use boring package not global var

Align with the upstream to use `boring` as a name for a package. No functional changes.

* new: name aliasing

Create u_alias.go to hold any alias names created by version upgrades or other necessary changes (e.g., upstream breaking change) to prevent further breaking the API.

u_alias.go

@@ -0,0 +1,12 @@
+package tls
+
+// This file contains all the alias functions, symbols, names, etc. that
+// was once used in the old version of the library.  This is to ensure
+// backwards compatibility with the old version of the library.
+
+// TLS Extensions
+
+// UtlsExtendedMasterSecretExtension is an alias for ExtendedMasterSecretExtension.
+//
+// Deprecated: Use ExtendedMasterSecretExtension instead.
+type UtlsExtendedMasterSecretExtension = ExtendedMasterSecretExtension