ICU-22953 MF2: Allow unpaired surrogates in text and quoted literals

[catamorphism]

In a previous version of the spec, unpaired surrogates in text or quoted literals were an error.
A recent PR, unicode-org/message-format-wg#906 , made them allowable. This PR changes ICU4C to be consistent with the spec.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22953
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[catamorphism]

Note: another PR, #3236 , changes the same parser code in order to use a UnicodeSet, but I'd rather keep this change separate and then rebase that PR after this one lands.