Set cmake minimum to 3.18

[BenWu]

Cmake 4.0.0 removed compatibility for versions <3.5: https://cmake.org/cmake/help/latest/release/4.0.html#deprecated-and-removed-features. I updated recently so build fails for me. Setting to 3.18 which is the version in the debian docker image.

Checklist for reviewer:

• Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• If adding a new field, the field should have a description (see Bug 1652842 - Add a blocklist field so the environment schema works with pre- and post-bug-1647225 data #576 for an example)
• If coming from a fork, run integration tests: ./.github/push-to-trigger-integration <username>:<branchname>

For glean changes:

• Update templates/include/glean/CHANGELOG.md

For modifications to schemas in restricted namespaces (see CODEOWNERS):

• Follow the change control procedure

[dataops-ci-bot]

Integration report for "Update version in readme"

bq_schema_960775ca9-cfb17fe4b.diff

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compact_schema_960775ca9-cfb17fe4b.diff

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