[milvus] Update default etcd image 3.5.21-r2 and bump chart version to 4.2.56

[0xkc0de]

Bump etcd #17

Checklist

• Chart Version bumped
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [mychartname])
• PR only contains changes for one chart

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: karthiknathan06
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[sre-ci-robot]

Welcome @karthiknathan06! It looks like this is your first PR to zilliztech/milvus-helm 🎉

[0xkc0de]

/assign @yellow-shine

[0xkc0de]

/unassign @yellow-shine

[haorenfsa]

/assign @zhuwenxing please help test

[haorenfsa]

@zhuwenxing how's the test going?

[0xkc0de]

Noticed there are conflicts now while the PR is under testing. Just wanted to check, would you like me to rebase it at this point?
@haorenfsa