[cpanato]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Revives #3627, rebased on master with the review feedback addressed.

Adds cmd/patch-release-notification, an AWS Lambda that runs once a day, reads the patch release schedule, and emails the community through SES when a cherry-pick deadline is DAYS_TO_ALERT days away. It runs in mock mode by default, which sends only to the sender address.

Changes since #3627:

• Handler takes no event payload, matching the EventBridge cron trigger
• Deadline is matched on calendar days in UTC, so the run time no longer shifts the day
• Moved to aws-sdk-go-v2 / sesv2 and dropped gomail, envconfig and yaml.v3
• Email uses html/template; the schedule fetch requires https, has a timeout and checks the response
• Added links to PRs that meet all cherry-pick criteria, as suggested in review
• Added unit tests

The schedule-builder model types move to cmd/schedule-builder/model so they can be shared.

Infra: kubernetes/k8s.io#10093 (replaces kubernetes/k8s.io#6853).

Merge order: this PR first, then kubernetes/k8s.io#10093. Nothing is deployed on merge. After both merge: apply the CloudFormation bootstrap, push the image built from master with ko, set image_uri to its digest in k8s.io, and run terraform apply. It starts in mock mode; no_mock = true comes in a later k8s.io PR, after SES production access is granted.

Which issue(s) this PR fixes:

Fixes #2174

Does this PR introduce a user-facing change?

Add an AWS Lambda function that sends patch release cherry-pick deadline reminders.

[kubernetes-prow]

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[cpanato]

/assign @saschagrunert @xmudrii @puerco @Verolop
cc @kubernetes/release-engineering