display announcement text as mfm block

What does this MR do?

display announcement text as mfm block (so bidi works correctly)

the announcements.vue page already does that; while reviewing !955 I noticed I had missed this line when I added all the :isBlock

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requested review from @fEmber and @Marie

added bugfrontend label

approved this merge request

approved this merge request

mentioned in commit 337b3524

merged