fix: add index on reported_at column of reports table

[satishrudderstack]

Description

• Adding index on reported_at on reports table which is used by reporting queries

Notion Ticket

• https://www.notion.so/rudderstacks/Reports-Flush-Issue-from-Rudder-Server-Instance-9-on-prousmtusmt-c9592bd2b2b74d05a9e083ad8baa7988

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.19 ⚠️

Comparison is base (cfee014) 68.29% compared to head (8fc345d) 68.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3606      +/-   ##
==========================================
- Coverage   68.29%   68.11%   -0.19%     
==========================================
  Files         318      318              
  Lines       50382    50382              
==========================================
- Hits        34408    34316      -92     
- Misses      13749    13840      +91     
- Partials     2225     2226       +1     

see 11 files with indirect coverage changes

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