JiraOnPremOAuth: Supports Record<string, string[]> which is impossible

The following functions indicate that the class was developed for a query param which might contain a key with multiple values.

However, based on the caller functions, this case is impossible. Every defined key will have a `string` value – not `string[]`.

Is there something missing? Are incoming values supposed to be split on a certain character?

## Code
https://github.com/matrix-org/matrix-hookshot/blob/1e8a112a28d116319e28de85bedf91b509012e94/src/Jira/oauth/OnPremOAuth.ts#L170-L184

https://github.com/matrix-org/matrix-hookshot/blob/1e8a112a28d116319e28de85bedf91b509012e94/src/Jira/oauth/OnPremOAuth.ts#L188-L197

Read src/Jira/oauth/OnPremOAuth.ts at lines 170-197 and trace the caller functions that provide the query parameters. Determine whether the string[] type is reachable or whether incoming values need defined splitting; done means the type and handling match the actual caller behavior.

Written by the indexing model from the issue text.