ci: pin the mongo sidecar image to 7.0

[wyattwalter]

Description

Every Cypress shard scheduled on a refreshed GitHub runner currently fails during setup: the mongo sidecar (unpinned, so mongo:latest = 8.x) refuses to start with

MongoDB cannot start: Linux kernel versions 6.19 and newer has a known incompatibility with this version of MongoDB. See https://jira.mongodb.org/browse/SERVER-121912

The incompatibility is between MongoDB 8.x's bundled TCMalloc and Linux kernels 6.19–7.0.13; the refreshed runner image carries an affected kernel, so shards fail or pass depending on which runner they land on (observed: 42 of ~60 shards failing on appsmith-ee#9672). MongoDB refuses to start by design; the resolution on affected hosts is a kernel ≥7.0.14, which we don't control on hosted runners.

This pins the sidecar to mongo:7.0, which does not bundle the affected allocator and is well within the supported range (server minimum is 5.0). All six workflows that declared the unpinned image are updated: ci-test-hosted, ci-test-playwright, ci-test-custom-script, ci-test-limited, ci-test-limited-with-count, external-pr-validation.

Revisit the pin when a MongoDB 8.x image ships with the re-vendored allocator or the runner kernels move past 7.0.13.

Automation

CI-workflow-only change; the Cypress suite is what this fixes, and it cannot run to green from an unmerged workflow-only branch on affected runners.

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