chore(deps): update postgres docker tag to v18

[renovate]

This PR contains the following updates:

Package	Update	Change	Age	Confidence
postgres	major	17-alpine → 18-alpine

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Configuration

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Snapshot Warnings

⚠️: No snapshots were found for the head SHA f235083.

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Scanned Files

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[sentry]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
664	2	662	36

View the full list of 2 ❄️ flaky test(s)

tests/cache/test_backend.py::TestValkeyBackend::test_setex_called_when_ttl_sec_provided

Flake rate in main: 100.00% (Passed 0 times, Failed 2 times)

Stack Traces | 0.009s run time

tests/cache/test_backend.py:162: in test_setex_called_when_ttl_sec_provided
    assert args[2] == "v"
E   assert '"v"' == 'v'
E     
E     #x1B[0m#x1B[91m- v#x1B[39;49;00m#x1B[90m#x1B[39;49;00m
E     #x1B[92m+ "v"#x1B[39;49;00m#x1B[90m#x1B[39;49;00m

tests/cache/test_backend.py::TestValkeyBackend::test_string_value_stored_as_is

Flake rate in main: 100.00% (Passed 0 times, Failed 2 times)

Stack Traces | 0.006s run time

tests/cache/test_backend.py:128: in test_string_value_stored_as_is
    assert mock_client.set.call_args[0][1] == "plain"
E   assert '"plain"' == 'plain'
E     
E     #x1B[0m#x1B[91m- plain#x1B[39;49;00m#x1B[90m#x1B[39;49;00m
E     #x1B[92m+ "plain"#x1B[39;49;00m#x1B[90m#x1B[39;49;00m
E     ? +     +#x1B[90m#x1B[39;49;00m

To view more test analytics, go to the [Prevent Tests Dashboard](https://All Things Linux.sentry.io/prevent/tests/?preventPeriod=30d&integratedOrgName=allthingslinux&repository=tux&branch=renovate%2Fmajor-18-docker)

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