Rename function that replaces variables in When Expressions

[jerop]

Changes

A member function of WhenExpressions should not have reference the object in the function name. This change renames the function ReplaceWhenExpressionsVariables to ReplaceVariables.

/kind cleanup

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[jerop]

/retest

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lbernick

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[Yongxuanzhang]

/lgtm