Breaking oas rule changes

[philsturgeon]

With v5.0 coming, now is the time to make changes to the OpenAPI ruleset.

1. example-value-or-externalValue make it recommended (spec says The value field and externalValue field are mutually exclusive.).

2. no-eval-in-markdown we should recommend this as a safety precaution, people can turn it off if they want eval in there.

3. openapi-tags we should make this recommended

4. operation-summary-formatted delete it. Summary is meant to be shorter than description, so is normally used for title case not sentences. That's usually the description.

5. model-description lets delete it, ive never written one and we currently dont elevate models to a point where this data is used anyway

6. valid-example oops we split this into multiple rules and forgot to update the docs 😅 (while you're in there extends: "spectral: oas3" has a space in it in the oas3 description)

7. the remaining ones in oas2 and oas3 will need to be turned into single rules which effect both formats if they share the same given, or made into two seperate rules if they do not.

Let's make sure the docs are totally up to date with the new reality of the rulesets and list this all in the migration guide.

[philsturgeon]

@P0lip @marbemac i think this (and #689) will make for a great v5.0 release. Cleaning up tech debt, simplifying docs, and removing some rules which people at the workshop and in general thought were a bit odd.