chore(oidc): graduate webkey to stable (#10122)

# Which Problems Are Solved

Stabilize the usage of webkeys.

# How the Problems Are Solved

- Remove all legacy signing key code from the OIDC API
- Remove the webkey feature flag from proto
- Remove the webkey feature flag from console
- Cleanup documentation

# Additional Changes

- Resolved some canonical header linter errors in OIDC
- Use the constant for `projections.lock` in the saml package.

# Additional Context

- Closes #10029
- After #10105
- After #10061

internal/command/key_pair.go

@@ -13,31 +13,6 @@ import (
 	"github.com/zitadel/zitadel/internal/repository/keypair"
 )
 
-func (c *Commands) GenerateSigningKeyPair(ctx context.Context, algorithm string) error {
-	privateCrypto, publicCrypto, err := crypto.GenerateEncryptedKeyPair(c.keySize, c.keyAlgorithm)
-	if err != nil {
-		return err
-	}
-	keyID, err := c.idGenerator.Next()
-	if err != nil {
-		return err
-	}
-
-	privateKeyExp := time.Now().UTC().Add(c.privateKeyLifetime)
-	publicKeyExp := time.Now().UTC().Add(c.publicKeyLifetime)
-
-	keyPairWriteModel := NewKeyPairWriteModel(keyID, authz.GetInstance(ctx).InstanceID())
-	keyAgg := KeyPairAggregateFromWriteModel(&keyPairWriteModel.WriteModel)
-	_, err = c.eventstore.Push(ctx, keypair.NewAddedEvent(
-		ctx,
-		keyAgg,
-		crypto.KeyUsageSigning,
-		algorithm,
-		privateCrypto, publicCrypto,
-		privateKeyExp, publicKeyExp))
-	return err
-}
-
 func (c *Commands) GenerateSAMLCACertificate(ctx context.Context, algorithm string) error {
 	now := time.Now().UTC()
 	after := now.Add(c.certificateLifetime)