build(npm): fix critical npm audit warnings

[Splaktar]

PR Checklist

Please check that your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

npm audit returns found 140 vulnerabilities (63 low, 44 moderate, 31 high, 2 critical).

Issue Number:
Fixes #11343. Fixes #11270.

What is the new behavior?

npm audit returns found 32 vulnerabilities (17 low, 5 moderate, 10 high).

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

This removes the unused gulp server task. Use gulp watch site --dev instead (as has been recommended for a long time).