fix: remove leading / from SSM parameter names in policy ARNs (#1146)

## Description

Using SSM parameter hierarchies (as described in
https://docs.aws.amazon.com/systems-manager/latest/userguide/sysman-paramstore-hierarchies.html)
results in an error. See #1128

Several workarounds exist, but using `trimprefix` makes them
superfluous. Having a `/` as first character results in the above
mentioned error.

## Verification

Checked the ARN of parameter `/test/test`. It showed up as
`arn:aws:ssm:eu-central-1:123456789012:parameter/test/test` in the
console. So no `//` at first place.

---------

Co-authored-by: kirkchong <kirk_ang_from.tp@tech.gov.sg>
Co-authored-by: Matthias Kay <matthias.kay@hlag.com>
Co-authored-by: Matthias Kay <github@matthiaskay.de>

.cspell.json

@@ -55,6 +55,7 @@
     "tfvars",
     "tmpfs",
     "tonumber",
+    "trimprefix",
     "trivy",
     "userns",
     "xanzy",

main.tf

@@ -615,14 +615,14 @@ data "aws_iam_policy_document" "ssm" {
           var.runner_gitlab.preregistered_runner_token_ssm_parameter_name,
           aws_ssm_parameter.runner_registration_token.name
         ]
-      ) : "arn:${data.aws_partition.current.partition}:ssm:${data.aws_region.current.name}:${data.aws_caller_identity.current.account_id}:parameter/${name}"
+      ) : "arn:${data.aws_partition.current.partition}:ssm:${data.aws_region.current.name}:${data.aws_caller_identity.current.account_id}:parameter/${trimprefix(name, "/")}"
     ]
   }
 
   statement {
     actions = ["ssm:PutParameter"]
     resources = [
-      "arn:${data.aws_partition.current.partition}:ssm:${data.aws_region.current.name}:${data.aws_caller_identity.current.account_id}:parameter/${aws_ssm_parameter.runner_registration_token.name}"
+      "arn:${data.aws_partition.current.partition}:ssm:${data.aws_region.current.name}:${data.aws_caller_identity.current.account_id}:parameter/${trimprefix(aws_ssm_parameter.runner_registration_token.name, "/")}"
     ]
   }
 }