chore: prepare release v3.7.10

[mekilis]

Summary

• Bump the chart to 3.7.10.
• Default the Convoy image to v26.7.2 across the root chart and the agent/server subcharts.

Test plan

• Merge after the v26.7.2 image exists. This PR only changes default tags.
• helm lint . and helm template . --debug render with global.convoy.tag=v26.7.2.

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Note

Low Risk
Version and default image tag metadata only; no template or runtime logic changes. Risk is mainly pulling a chart before the v26.7.2 image is published unless installs override global.convoy.tag.

Overview
Prepares Helm release v3.7.10 by bumping chart and vendored agent/server subchart versions from 3.7.9 to 3.7.10, and setting appVersion and default Convoy image tags from v26.6.6 to v26.7.2.

Updates span the root Chart.yaml, values.yaml (global.convoy.tag), subchart Chart.yaml / values.yaml, regenerated Chart.lock, README badges and values table, and the Artifact Hub change note for the new default image.

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