fix: remove leading `/` from SSM parameter names in policy ARNs

[Kqirk]

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.

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