build(mergify): specify checks and include Snyk

[mantariksh]

Problem

• Mergify config does not specify each status check explicitly, against the recommendations of the docs
• Snyk.io security upgrades are not included in the mergify config, so they have to be approved and merged manually

Closes #1749

Solution

• Specify all status checks explicitly in mergify config
• Include a config rule to approve and major non-major Snyk security upgrades

Screenshots

Taken from Mergify config editor.

Config passes for dependabot upgrades e.g. #2810

Config passes for Snyk security upgrades e.g. #2412

Note that the "potential rule" is because this PR was taken when our Snyk license was still under "Open Government Products", so the relevant GitHub branch protection doesn't pass. This should no longer be the case for newer PRs for which the license is under "FormSG".

Config does not pass for regular PRs e.g. #2776

[tshuli]

lgtm