Remove maximum length requirement for API key

[kengel100]

Is there a reason to specify max length for API key in integrations?
Eg https://github.com/OctopusDeploy/OctoTFS/pull/187/files and https://github.com/OctopusDeploy/octopus-jenkins-plugin/pull/98/files

If Core extends the API key length again these integrations will break again. (Currently, there aren't any plans to make it longer.)
We could consider adding data contracts and safety nets to Core, but the question came up as to why integrations rely on a specific length.

Note: other integrations may also have maximum length hardcoded.