Dependencies: upgrade gulp-eslint to 6.0.0

[musikele]

Type of change

• Bugfix

• Feature

• New bidder adapter

• Code style update (formatting, local variables)

• [ X ] Refactoring (no functional changes, no api changes)

• Build related changes

• CI related changes

• Does this change affect user-facing APIs or examples documented on http://prebid.org?

• Other

Description of change

In order to use javascript operator ??, we need to upgrade gulp-eslint.

Other information

The need for this PR came from LiveIntent PR 8960

Example output:

➜  Prebid.js git:(upgrade_eslint) ✗ npx gulp lint 
[15:05:26] Using gulpfile ~/Documents/code/magnite/Prebid.js/gulpfile.js
[15:05:26] Starting 'lint'...
[15:06:09] Finished 'lint' after 43 s

When something is wrong, errors are still being reported:

➜  Prebid.js git:(upgrade_eslint) ✗ npx gulp lint
[15:08:03] Using gulpfile ~/Documents/code/magnite/Prebid.js/gulpfile.js
[15:08:03] Starting 'lint'...
[15:08:46] 
/Users/mnasti/Documents/code/magnite/Prebid.js/src/prebid.js
  27:5  error  'evil' is defined but never used  no-unused-vars

✖ 1 problem (1 error, 0 warnings)

[15:08:46] 'lint' errored after 43 s
[15:08:46] ESLintError in plugin "gulp-eslint"
Message:
    Failed with 1 error
Details:
    domainEmitter: [object Object]
    domainThrown: false