changed refresh policy from IMMEDIATE TO WAIT_UNTIL in Alert Service class

[eirsep]

Although there are many other instances of Refresh Policy IMMEDIATE being used in persisting various resources in alerting plugin, Indexing alerts is one aspect which requires the WAIT_UNTIL refresh policy to be set because chained alert triggers rely on availability of AUDIT alerts.

Hence I am making a change only in this class. Other instances of refresh policy changes will need to be evaluated and taken up in a separate PR.

[codecov]

Codecov Report

Merging #1075 (d20d01e) into main (09911d2) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #1075      +/-   ##
============================================
+ Coverage     67.71%   67.75%   +0.03%     
  Complexity      105      105              
============================================
  Files           160      160              
  Lines         10343    10343              
  Branches       1522     1522              
============================================
+ Hits           7004     7008       +4     
+ Misses         2674     2669       -5     
- Partials        665      666       +1     

Files Changed	Coverage Δ
...ain/kotlin/org/opensearch/alerting/AlertService.kt	81.58% <100.00%> (ø)

... and 5 files with indirect coverage changes