improvement: maintenance+cleanup+fix (#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.

internal/boring/notboring.go

@@ -0,0 +1,16 @@
+package boring
+
+import (
+	"crypto/cipher"
+	"errors"
+)
+
+const Enabled bool = false
+
+func NewGCMTLS(_ cipher.Block) (cipher.AEAD, error) {
+	return nil, errors.New("boring not implemented")
+}
+
+func Unreachable() {
+	// do nothing
+}