Add regexp validation on ContainerExpirationPolicy

What does this MR do?

Validates the regular expressions saved within the Container Expiration Policy object. The attributes to be validated are name_regex and name_regex_keep.

For more details, see #216088 (comment 355336634)

With this MR, those columns/attributes are now validated with

validates :name_regex, untrusted_regexp: true
validates :name_regex_keep, untrusted_regexp: true

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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