remove DISABLED value from 'zowe.verifyCertificates' property

[nxhafa]

Describe your pull request here:

Update the v2.x documentation to remove the DISABLED option from zowe.verifyCertificates property in zowe.yaml.
Only the STRICT and NONSTRICT options will be available.

List the file(s) included in this PR:
Updated files are located in docs-site/versioned_docs/version-v2.18.x

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