[Security Solution][Detection Engine] Suppression fields combobox is case-insensitive

[vitaliidm]

Describe the bug: Suppression fields combobox is case-insensitive

Kibana/Elasticsearch Stack version: 8.6 + (since suppression introduced)

Steps to reproduce:

1. Create index with 2 equal fields: one capitalized, one not
2. Select one of the fields in Group by fields of suppression options
3. Another field is not available to select

Any additional context (logs, chat logs, magical formulas, etc.):

By default, combobox is case-insenstive: https://elastic.github.io/eui/#/forms/combo-box#case-sensitive-matching

But since ES fields case-sensitive, it could lead to the described above issue

[elasticmachine]

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

[vitaliidm]

cc: @yctercero