chore: set Dependabot update intervals to monthly

[Matt (matt-evervault)]

Why

Slow down Dependabot's update cadence from daily to monthly for all three ecosystems (github-actions, gradle, npm) configured in .github/dependabot.yml.

How

Changed schedule.interval from "daily" to "monthly" for all three ecosystem entries. Nothing else in the file was touched — open-pull-requests-limit: 0 is left in place on all three entries, which already suppresses version-update PRs entirely, so this interval change only affects the cadence of security-update PRs (which ignore open-pull-requests-limit).

Ecosystem check

• github-actions: .github/workflows/*.yml exists (codeql.yml, release.yml, test.yml), so directory: "/" is representative for this ecosystem.
• gradle: The root build.gradle.kts lives at /, but the project is multi-module — evervault-cages/build.gradle.kts, evervault-core/build.gradle.kts, evervault-enclaves/build.gradle.kts, evervault-inputs/build.gradle.kts, examples/shared/build.gradle.kts, and sampleapplication/build.gradle.kts all live outside the root and are not directly covered by directory: "/". Dependabot's gradle updater does resolve module structure via settings.gradle.kts, so root-only config may still cover the whole project, but this wasn't verified end-to-end — flagging for awareness, not changing as part of this PR.
• npm: A root package.json exists (private package with @changesets/cli as the only devDependency, used for versioning scripts), so the npm entry is real, not vestigial.

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