Update Stable overlay and Changelogs for 1.2.1 driver image

[saikat-royc]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
The internal prow tests for prow-staging-rc-{k8minor} with image v1.2.1-rc1 looks good. Hence submitting this PR to create a new release v1.2.1 for the driver image binary
.This PR updates the stable overlays driver manifest to use the yet to be release v1.2.1-gke.0 image, and the necessary changelogs/readme updates.

Link to the test grid: https://testgrid.corp.google.com/gcp-compute-persistent-disk-csi-driver
Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

FYI once this PR is merged the cherry-pick will not be a clean one, since the release-1.2 does not have the per minor version overlays. In that cherry-pick I will manually update the stable/image.yaml file. In future release branches (say release-1.3), the overlays per k8s minor version will be picked up.

Next steps: when this PR is merged, cherry pick to release-1.2 branch and create v1.2.1 tag. Promote the image manually to gcr.io/gke-release

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

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This pull-request has been approved by: saikat-royc

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[saikat-royc]

/assign @saad-ali @mattcary

[saikat-royc]

/retest

[mattcary]

/lgtm