Update to OpenAPI version 3.1

[cookeac]

Update to OpenAPI 3.1

The following changes have been made to all OpenAPI files in the url-schemes folder:

• Updated to "openapi": "3.1.0"
• Updated to version 1.4 (ahead of the ADE 1.4 release)
• Updated all tags to ADE-1.4-xxxx

The following changes have been made to JSON Schema files in the types and resources folders where "nullable": true was previously specified:

• replace "type": "string" with "type": ["string", "null"]
• replace "type": "number" with "type": ["number", "null"]
• replace "type": "integer" with "type": ["integer", "null"]
• replace "type": "boolean" with "type": ["boolean", "null"]
• replace "type": "array" with "type": ["array", "null"]

Where a complex type (imported with "$ref" was previously nullable):

• If the type was an object, add "type": ["object", "null"] before the $ref
• If the type was a string (an enumeration or an icarDateTimeType), add "type": ["string", "null"] before the $ref

These last changes appear in examples online, but they are the ones that are most likely to interrupt code generation. Please test.

Resolves #239

[cookeac]

A further note - until it is merged, this PR will fail Speccy validation. This is because Speccy is deprecated and does not support OpenAPI 3.1. When this PR is merged, it will remove the GitHub action for Speccy, leaving us with Spectral validation.

[cookeac]

We agreed (meeting 2024-04-04) that we could merge this once #451 is addressed with a PR, as @AlexeyHardCode has tested code generation with C# at least.