Breaking change: Deprecate legacy shell escaping and quoting runner shell executor

Related issue: gitlab-org/gitlab#406679 (closed)

I create this issue to investigate since this change is related to runner arguments.

The runner's legacy escape sequence mechanism to handle variable expansion implements a sub-optimal implementation of Ansi-C quoting. This method means that the runner would expand arguments included in double quotes. As of 15.11, we are deprecating the legacy escaping and quoting methods in the runner shell executor.

From spreadsheet/impact: Low