Clarify the support policy

[danyalaytekin]

See comment:

• Update Node.js support to 18-20 from 12-16, dependencies, documentation, some practices pa11y-webservice-client-node#19 (comment)

Perhaps we could also centralise the refined description (not necessarily the tables) and make it shareable, if it will be the same policy for each project, as also discussed by:

• Managing documentation for the whole project #16

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Now tracking the rollout:

• Release pa11y@7 pa11y#681
• Version 3.1.0 pa11y-ci#226
• Release pa11y-lint-config@3 pa11y-lint-config#8
• Release [email protected] pa11y-webservice#148
• Release pa11y-webservice-client-node@4 pa11y-webservice-client-node#24
• Release [email protected] pa11y-dashboard#320

[danyalaytekin]

We've agreed this wording:

When we release a new major version we will continue to support the previous major version for 6 months. This support will be limited to fixes for critical bugs and security issues.