improve merge claims behavior

[pamapa]

• In order to allow "refresh userinfo data on demand" refresh userinfo data on demand #877 merging claims must be adapted in way way that we can do it multiple times.
• Changing the type of string to array by default (to preserve the previous different value) seems not a good idea. In case this is needed by some authz servers, we could add a special settings to have this behavior back (see #881#issuecomment-1425834324). mergeClaimsStrategy?: "replace" | "merge"]
• drop misleading mergeClaims setting, we always merge ...

[pamapa]

@Badisi I will take the code from @marcoreni without the legacy fallback as a starting point for v3. See https://github.com/authts/oidc-client-ts/pull/881/files#diff-f13fa4d03ace5bba3b296851ab7dde13eafcc8e3643928cf369dd465af004d29R94