[RAC][Observability] Stop indexing severity in Uptime

[mgiota]

📝 Summary

At the moment Uptime indexes severity under kibana.alert.severity with values critical and warning. We should stop indexing these values for following reasons:

• We don't display the severity value in the Observability Alerts table anyway. Severity column is hidden ([RAC][Observability] temporarily hide severity column #109004)
• We need to clarify first the severity levels across Observability before we start populating the severity value. Related tickets https://github.com/elastic/observability-product/issues/29 and [Meta][RAC] add alert severity levels in Observability applications #97951

[mgiota]

@jasonrhodes As discussed I wanted to stop indexing the severity value for APM as well as part of this ticket. I kept indexing the severity for APM because it is currently used in two places:

• as an annotation in a chart
• it is used to construct the reason message in Observability alerts table

The severity values for APM are being calculated reading a severity score and they can be critical, major, minor, warning, low or unknown https://github.com/elastic/kibana/blob/main/x-pack/plugins/ml/common/util/anomaly_utils.ts#L131

@marshallmain If I remember correctly are these the same values you use in Security for the severity value?