Change prefix from SECRETS_ to SECRET_ for consistency with the `secret()` function
### Feature description

Given that this prefix is added for each (singular) secret, SECRET_ prefix seems more natural and easier to understand/remember. 

Also, the `secret()` function currently can only retrieve one secret at a time so plural SECRETS_ can be misleading. 

cc @brian-mulier-p 🙏
build.gradle core/src/main/java/io/kestra/core/secret/SecretService.java
build.gradle core/src/main/java/io/kestra/core/secret/SecretService.java
diff --git a/build.gradle b/build.gradle
index cfe5455df3..3e4832e055 100644
--- a/build.gradle
+++ b/build.gradle
@@ -169,8 +169,8 @@ subprojects {
         systemProperty 'user.language', 'en'
         systemProperty 'user.country', 'US'
 
-        environment 'SECRETS_MY_SECRET', "{\"secretKey\":\"secretValue\"}".bytes.encodeBase64().toString()
-        environment 'SECRETS_NON_B64_SECRET', "some secret value"
+        environment 'SECRET_MY_SECRET', "{\"secretKey\":\"secretValue\"}".bytes.encodeBase64().toString()
+        environment 'SECRET_NON_B64_SECRET', "some secret value"
     }
 
     testlogger {
diff --git a/core/src/main/java/io/kestra/core/secret/SecretService.java b/core/src/main/java/io/kestra/core/secret/SecretService.java
index 05b9e859e8..5cb9bbd98a 100644
--- a/core/src/main/java/io/kestra/core/secret/SecretService.java
+++ b/core/src/main/java/io/kestra/core/secret/SecretService.java
@@ -16,7 +16,7 @@
 @Singleton
 @Slf4j
 public class SecretService {
-    private static final String SECRET_PREFIX = "SECRETS_";
+    private static final String SECRET_PREFIX = "SECRET_";
     private Map<String, String> decodedSecrets;
 
     @PostConstruct
@@ -40,7 +40,7 @@ public String findSecret(String namespace, String key) throws IOException, Illeg
         return Optional
             .ofNullable(decodedSecrets.get(key.toUpperCase()))
             .orElseThrow(() -> new IllegalVariableEvaluationException("Unable to find secret '" + key + "'. " +
-                "You should add it in your environment variables as 'SECRETS_" + key.toUpperCase() +
+                "You should add it in your environment variables as '" + SECRET_PREFIX + key.toUpperCase() +
                 "' with base64-encoded value."
             ));
     }
