e2e test: add schema version verification in mix_version_test.

[siyuanfoundation]

#17361
#17752

Verify storage version is consistent with expected at the end of tests.
Since storageVersion is the only field added in 3.6, there is no need to check other buckets.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[siyuanfoundation]

/retest

[siyuanfoundation]

/retest

[siyuanfoundation]

cc @ahrtr

[siyuanfoundation]

/retest

[ahrtr]

The existing schemaChanges isn't flexible. Propose to add the following changes, and we can construct the the schemaChanges based on the new schemaChangeMappings.

func GetNewFieldsInVersion(ver semver.Version) []NewField {
	if newFields, found := schemaChangeMappings[ver]; found {
		return newFields
	}

	return nil
}

type NewField struct {
	bucket       backend.Bucket
	field        []byte
	defaultValue []byte
}

var (
	schemaChangeMappings = map[semver.Version][]NewField{
		version.V3_6: {
			{Meta, MetaStorageVersionName, emptyStorageVersion},
		},
	}

	......
)

[ahrtr]

LGTM

Thanks

[ahrtr]

cc @serathius to take a look