[chore][installer-script-test] Upgrade minimum version of Node.js to v18 in testing

[crobert-1]

Description:

Test failure output from linux-installer-script-test:

npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: '@splunk/[email protected]',
npm WARN EBADENGINE   required: { node: '>=18' },
npm WARN EBADENGINE   current: { node: 'v16.20.2', npm: '8.19.4' }
npm WARN EBADENGINE }

Failure link

NOTE: v16 has been EOL since August 2023. v18 is also EOL, but I think it makes sense to upgrade one major version at a time for now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.01%. Comparing base (8a58ae5) to head (1906aca).
Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (34.01%) is below the target coverage (40.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (8a58ae5) and HEAD (1906aca). Click for more details.

HEAD has 13 uploads less than BASE

Flag	BASE (8a58ae5)	HEAD (1906aca)
	35	22

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6235       +/-   ##
===========================================
- Coverage   44.35%   34.01%   -10.34%     
===========================================
  Files         390      390               
  Lines       26986    26986               
===========================================
- Hits        11969     9179     -2790     
- Misses      14171    17181     +3010     
+ Partials      846      626      -220     

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