Replace EachSequential and EachParallel with ForEach

[wrussell1999]

Description

A few places in the docs still reference EachSequential and EachParallel / use it in examples. We need to replace these and link to ForEach instead.

Places that need fixing (but doesn't necessarily mean this is everything - add as you find stuff)

• https://kestra.io/docs/best-practices/flows#task-in-the-same-execution
• https://kestra.io/docs/workflow-components/outputs#specific-outputs-for-dynamic-tasks
• https://kestra.io/docs/workflow-components/variables#can-i-use-nested-variables
• https://kestra.io/docs/tutorial/flowable
• https://kestra.io/docs/migration-guide/0.11.0/templates#templates-%EF%B8%8F
• https://kestra.io/docs/concepts/namespace-files
• adjust all Trigger plugin examples to use ForEach instead of EachSequential  #1974
  are using this deprecated plugin - we should update all those plugin examples - @wrussell1999
• https://kestra.io/docs/expressions/expression-usage

(check to see if any are missing)

We also need to redo the Flowable Page and remove EachParallel and EachSequential.

• Replace references - @shrutimantri
• Reword page - @wrussell1999

[anna-geller]

for the last one about Trigger, would be cool to take time to clean up the Trigger documentation so that it's easy to understand - see example kestra-io/plugin-azure@35afd63

[wrussell1999]

@shrutimantri can you go through all of these and update to ForEach?

• https://github.com/search?q=org%3Akestra-io%20EachSequential&type=code
• https://github.com/search?q=org%3Akestra-io%20EachParallel&type=code