Add some improvements to the loadtesting environment (#11564)

The default max open conns (50) was causing issues with small DB
instances (`db.t4g.medium`) in loadtesting (we run 10 fleet instances,
and 500 was over the limit number of connections for that instance
type).

Also upgrading Redis version to a more recent version that users are
most likely using.

infrastructure/loadtesting/terraform/ecs.tf

@@ -140,6 +140,10 @@ resource "aws_ecs_task_definition" "backend" {
             name  = "FLEET_MYSQL_ADDRESS"
             value = "${module.aurora_mysql.rds_cluster_endpoint}:3306"
           },
+          {
+            name  = "FLEET_MYSQL_MAX_OPEN_CONNS"
+            value = "5"
+          },
           {
             name  = "FLEET_MYSQL_READ_REPLICA_USERNAME"
             value = module.aurora_mysql.rds_cluster_master_username
@@ -152,6 +156,10 @@ resource "aws_ecs_task_definition" "backend" {
             name  = "FLEET_MYSQL_READ_REPLICA_ADDRESS"
             value = "${module.aurora_mysql.rds_cluster_reader_endpoint}:3306"
           },
+          {
+            name  = "FLEET_MYSQL_READ_REPLICA_MAX_OPEN_CONNS"
+            value = "5"
+          },
           {
             name  = "FLEET_REDIS_ADDRESS"
             value = "${aws_elasticache_replication_group.default.primary_endpoint_address}:6379"

infrastructure/loadtesting/terraform/redis.tf

@@ -7,7 +7,7 @@ resource "aws_elasticache_replication_group" "default" {
   replication_group_id       = "${local.prefix}-redis"
   num_cache_clusters         = 3
   node_type                  = var.redis_instance_type
-  engine_version             = "5.0.6"
+  engine_version             = "6.2"
   port                       = "6379"
   snapshot_retention_limit   = 0
   automatic_failover_enabled = true
@@ -20,7 +20,7 @@ resource "aws_elasticache_replication_group" "default" {
 
 resource "aws_elasticache_parameter_group" "default" { #tfsec:ignore:aws-vpc-add-description-to-security-group-rule
   name   = "${local.prefix}-redis"
-  family = "redis5.0"
+  family = "redis6.x"
 
   parameter {
     name  = "client-output-buffer-limit-pubsub-hard-limit"