ICU-22740 Add missing template arguments in MF2 parser

[catamorphism]

This fixes a clang-17 -Wctad-maybe-unsupported warning.

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22740
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [ ] Issue accepted (done by Technical Committee after discussion)
• [x] Tests included, if applicable
• [x] API docs and/or User Guide docs changed or added, if applicable

ALLOW_MANY_COMMITS=true

[catamorphism]

cc @roubert @echeran

[catamorphism]

Hm, I didn't realize before that there were configurations that used -Werror without also using -Wall -Wextra, maybe this'll be a bit easier to maintain if only those get the new warning added.

Fixed in b849e63

[roubert]

Fixed in b849e63

Now you got it backwards, -Wctad-maybe-unsupported should go together with -Werror -Wall -Wextra.

[catamorphism]

Now you got it backwards, -Wctad-maybe-unsupported should go together with -Werror -Wall -Wextra.

Oops. Should be fixed now.

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