[BUGFIX] Patch issue with SlackNotificationAction header rendering

[cdkini]

Header doesn't seem to support mrkdwn syntax so changing to plain_text

• Description of PR changes above includes a link to an existing GitHub issue
• PR title is prefixed with one of: [BUGFIX], [FEATURE], [DOCS], [MAINTENANCE], [CONTRIB]
• Code is linted - run invoke lint (uses ruff format + ruff check)
• Appropriate tests and docs have been updated

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Name	Link
🔨 Latest commit	69238ed
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.38%. Comparing base (97902d9) to head (69238ed).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9903   +/-   ##
========================================
  Coverage    77.38%   77.38%           
========================================
  Files          493      493           
  Lines        42037    42037           
========================================
  Hits         32532    32532           
  Misses        9505     9505           

Flag	Coverage Δ
3.10	63.92% <100.00%> (ø)
3.11	63.92% <100.00%> (ø)
3.11 athena or clickhouse or openpyxl or pyarrow or project or sqlite or aws_creds	53.75% <0.00%> (ø)
3.11 aws_deps	44.75% <0.00%> (ø)
3.11 big	54.20% <0.00%> (ø)
3.11 databricks	45.89% <0.00%> (ø)
3.11 filesystem	59.15% <0.00%> (ø)
3.11 mssql	48.73% <0.00%> (ø)
3.11 mysql	48.78% <0.00%> (ø)
3.11 postgresql	52.65% <0.00%> (ø)
3.11 snowflake	46.53% <0.00%> (+<0.01%)	⬆️
3.11 spark	56.28% <0.00%> (ø)
3.11 trino	50.71% <0.00%> (ø)
3.8	63.95% <100.00%> (ø)
3.8 athena or clickhouse or openpyxl or pyarrow or project or sqlite or aws_creds	53.76% <0.00%> (ø)
3.8 aws_deps	44.77% <0.00%> (ø)
3.8 big	54.21% <0.00%> (ø)
3.8 databricks	45.91% <0.00%> (ø)
3.8 filesystem	59.16% <0.00%> (ø)
3.8 mssql	48.71% <0.00%> (ø)
3.8 mysql	48.76% <0.00%> (ø)
3.8 postgresql	52.64% <0.00%> (ø)
3.8 snowflake	46.54% <0.00%> (?)
3.8 spark	56.24% <0.00%> (ø)
3.8 trino	50.70% <0.00%> (ø)
3.9	63.94% <100.00%> (ø)
cloud	0.00% <0.00%> (ø)
docs-basic	47.36% <0.00%> (ø)
docs-creds-needed	48.40% <0.00%> (ø)
docs-spark	47.07% <0.00%> (ø)

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