Remove specialist topic code from email alert API

[unoduetre]

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What

When all specialist topic pages have been archived and redirected, we need to remove the specialist topic related code from Email alert API

Why

Remove tech debt.

Trello ticket

Dependencies

The following PR has been extracted from the current one and needs to be merged first. The following PR fixes Pact tests.

[unoduetre]

Thanks Mat,

I've requested some changes, hopefully the comments makes sense?

I'd like to split this PR to make it easier for us to get the code reviewed, and make the intention of the work a bit easier to understand. I can see you extracted the pact tests into their own PR, but presumably that's not going to pass against a branch of gds-api-adapters without the SubsciberList factory being updated (which is still in this PR)?

Can I suggest we have one PR that removes topic as a valid tag for email alert api. This would contain your changes to

* lib/email_alert_criteria.rb

* lib/valid_tags.rb

* spec/validators/tags_validator_spec.rb

* spec/lib/email_alert_criteria_spec.rb

And then have a lower risk PR, which includes the remaining changes to the pact tests and factories and all specs, swapping out references to topics to taxons. That PR could go first, and would be about removing legacy references to specialist_topics. Ie not application code.

I think we can safely move all the tests to a separate PR. Can I move them to this PR (I will change the name and the description of the PR accordingly), so we would have 1 PR dealing with tests and 1 PR applying the actual change?