Add index for project_id and lower(name) for timeline event tags table

What does this MR do and why?

This MR adds a new index with project_id and lower(name) to incident_management_timeline_event_tags.

Migrations

Add text limit to new col

UP

main: == 20221109100217 AddTextLimitToTagsKey: migrating ============================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- current_schema()
main:    -> 0.0007s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("ALTER TABLE incident_management_timeline_event_tags\nADD CONSTRAINT check_060ffe253e\nCHECK ( char_length(key) <= 255 )\nNOT VALID;\n")
main:    -> 0.0009s
main: -- current_schema()
main:    -> 0.0001s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- execute("ALTER TABLE incident_management_timeline_event_tags VALIDATE CONSTRAINT check_060ffe253e;")
main:    -> 0.0011s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20221109100217 AddTextLimitToTagsKey: migrated (0.0129s) ===================

DOWN

  main: == 20221109100217 AddTextLimitToTagsKey: reverting ============================
  main: -- transaction_open?()
  main:    -> 0.0000s
  main: -- transaction_open?()
  main:    -> 0.0000s
  main: -- execute("            ALTER TABLE incident_management_timeline_event_tags\n            DROP CONSTRAINT IF EXISTS check_060ffe253e\n")
  main:    -> 0.0007s
  main: == 20221109100217 AddTextLimitToTagsKey: reverted (0.0092s) ===================

How to set up and validate locally

1. Run the migrations and check relevant changes in db/structure.sql and database tables.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #373853 (closed)