Allow customer managed KMS key for backup SNS topic

[dms1981]

Addresses trivy findings in https://github.com/ministryofjustice/modernisation-platform/actions/runs/8044933763/job/21969343240

[github-actions]

TFSEC Scan Success

Show Output

```hcl

---

TFSEC will check the following folders:
.

---

Running TFSEC in .
Excluding the following checks: AWS089, AWS099, AWS009, AWS097, AWS018

======================================================
tfsec is joining the Trivy family

tfsec will continue to remain available
for the time being, although our engineering
attention will be directed at Trivy going forward.

You can read more here:
aquasecurity/tfsec#1994

timings
──────────────────────────────────────────
disk i/o 603.805µs
parsing 606.387195ms
adaptation 804.192µs
checks 12.131541ms
total 619.926733ms

counts
──────────────────────────────────────────
modules downloaded 1
modules processed 6
blocks processed 257
files read 32

results
──────────────────────────────────────────
passed 27
ignored 18
critical 0
high 0
medium 0
low 0

No problems detected!

tfsec_exitcode=0

</details> #### `Checkov Scan` Success
<details><summary>Show Output</summary>

```hcl

*****************************

Checkov will check the following folders:
.

*****************************

Running Checkov in .
Excluding the following checks: CKV_GIT_1
2024-02-26 08:49:05,875 [MainThread  ] [WARNI]  Failed to download module github.com/ministryofjustice/modernisation-platform-terraform-s3-bucket?ref=8688bc15a08fbf5a4f4eef9b7433c5a417df8df1:None (for external modules, the --download-external-modules flag is required)
terraform scan results:

Passed checks: 306, Failed checks: 0, Skipped checks: 85

github_actions scan results:

Passed checks: 132, Failed checks: 0, Skipped checks: 0


checkov_exitcode=0

CTFLint Scan Success

Show Output

*****************************

Setting default tflint config...
Running tflint --init...
Installing "terraform" plugin...
Installed "terraform" (source: github.com/terraform-linters/tflint-ruleset-terraform, version: 0.5.0)
tflint will check the following folders:
.

*****************************

Running tflint in .
tflint_exitcode=0

Trivy Scan

Show Output

[Khatraf]

LGTM