[Case] Markdown table is not correctly migrated to newer version of Kibana after 7.14

[dej611]

Kibana version: 7.15 onwards

Describe the bug:

When a case markdown comment contains a table the migration process halts with the following error:

kibana_7_15_0    | {"type":"log","@timestamp":"2021-11-23T15:58:43+00:00","tags":["error","savedobjects-service"],"pid":1220,"message":"Error: Cannot handle unknown node `table`\n    at Object.unknown (/usr/share/kibana/node_modules/mdast-util-to-markdown/lib/index.js:65:9)\n    at Object.one [as handle] (/usr/share/kibana/node_modules/zwitch/index.js:20:25)\n    at flow (/usr/share/kibana/node_modules/mdast-util-to-markdown/lib/util/container-flow.js:15:15)\n    at Object.root (/usr/share/kibana/node_modules/mdast-util-to-markdown/lib/handle/root.js:6:10)\n    at Object.one [as handle] (/usr/share/kibana/node_modules/zwitch/index.js:20:25)\n    at toMarkdown (/usr/share/kibana/node_modules/mdast-util-to-markdown/lib/index.js:38:20)\n    at compile (/usr/share/kibana/node_modules/remark-stringify/index.js:13:12)\n    at Function.stringify (/usr/share/kibana/node_modules/unified/index.js:337:12)\n    at stringifyComment (/usr/share/kibana/x-pack/plugins/cases/common/utils/markdown_plugins/utils.js:56:6)\n    at /usr/share/kibana/x-pack/plugins/cases/server/saved_object_types/migrations/index.js:101:56\n    at tryTransformDoc (/usr/share/kibana/src/core/server/saved_objects/migrations/core/document_migrator.js:553:22)\n    at migrateProp (/usr/share/kibana/src/core/server/saved_objects/migrations/core/document_migrator.js:635:22)\n    at applyMigrations (/usr/share/kibana/src/core/server/saved_objects/migrations/core/document_migrator.js:353:20)\n    at DocumentMigrator.transformAndValidate [as transformDoc] (/usr/share/kibana/src/core/server/saved_objects/migrations/core/document_migrator.js:293:22)\n    at /usr/share/kibana/src/core/server/saved_objects/migrations/core/document_migrator.js:101:16\n    at Immediate._onImmediate (/usr/share/kibana/src/core/server/saved_objects/migrations/core/migrate_raw_docs.js:158:17)"}

That is due to a change to the new remarkStringify plugin introduced in #96703 which since version 9 drop the support for markdown table ( release 9.0.0 connected with remark 13.0.0 which dropped table support in favour of a module split )

The migration problem does not happen up to 7.14.0 .

Steps to reproduce:

1. Create a 7.13 instance and add a table comment to a case
2. Upgrade the instance to 7.15
3. The migration process halts.

Expected behavior:

The migration process should successfully complete.

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[elasticmachine]

Pinging @elastic/security-threat-hunting-cases (Team:Threat Hunting:Cases)

[cnasikas]

cc @patrykkopycinski

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)