fix(docs): update Linux installation instructions for GPG key handling

Author: sandhilt

cmd/migrate/README.md

@@ -27,8 +27,8 @@ $ scoop install migrate
 ### Linux (*.deb package)
 
 ```bash
-$ curl -L https://packagecloud.io/golang-migrate/migrate/gpgkey | apt-key add -
-$ echo "deb https://packagecloud.io/golang-migrate/migrate/ubuntu/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/migrate.list
+$ curl -fsSL https://packagecloud.io/golang-migrate/migrate/gpgkey | sudo gpg --dearmor -o /etc/apt/keyrings/migrate.gpg
+$ echo "deb [signed-by=/etc/apt/keyrings/migrate.gpg] https://packagecloud.io/golang-migrate/migrate/ubuntu/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/migrate.list
 $ apt-get update
 $ apt-get install -y migrate
 ```