Update Terraform hashicorp/terraform to ~> 1.15

Author: renovate[bot]

packages/airnode-deployer/terraform/aws/backend.tf

@@ -1,5 +1,5 @@
 terraform {
-  required_version = "~> 1.4"
+  required_version = "~> 1.15"
 
   backend "s3" {
     key      = "terraform.tfstate"

packages/airnode-deployer/terraform/aws/providers.tf

@@ -6,7 +6,7 @@ terraform {
     }
   }
 
-  required_version = "~> 1.4"
+  required_version = "~> 1.15"
 }
 
 provider "aws" {

packages/airnode-deployer/terraform/gcp/backend.tf

@@ -1,5 +1,5 @@
 terraform {
-  required_version = "~> 1.4"
+  required_version = "~> 1.15"
 
   backend "gcs" {
   }

packages/airnode-deployer/terraform/gcp/providers.tf

@@ -6,7 +6,7 @@ terraform {
     }
   }
 
-  required_version = "~> 1.4"
+  required_version = "~> 1.15"
 }
 
 provider "google" {