Bump version.txt to 1.21.7

[josh-ferrell]

What type of PR is this?

Uncomment only one, leave it on its own line:

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

/kind cleanup

What this PR does / why we need it: Bump version.txt to 1.21.7 to trigger a new cloud-provider-aws build.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

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[josh-ferrell]

/retest

[dims]

/lgtm

[nckturner]

/lgtm /approve

[k8s-ci-robot]

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