Update NodeJS version to v20.19.0 for CI and Docs

[khanhtc1202]

What this PR does:

Update NodeJS version to v20.19.0 for CI and Docs

Why we need it:

Keep NodeJS version consistency across the project. This version is already being used in the controlplane Dockerfile, so it should not affect our users.

NOTE: The site is using NodeJS v24, which is required by Hugo, and this doesn't relate to the PipeCD codebase itself.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.48%. Comparing base (3f5ea20) to head (ccd3d20).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6704   +/-   ##
=======================================
  Coverage   29.48%   29.48%           
=======================================
  Files         593      593           
  Lines       63440    63440           
=======================================
  Hits        18706    18706           
  Misses      43289    43289           
  Partials     1445     1445           

Flag	Coverage Δ
.	23.29% <ø> (ø)
.-pkg-app-pipedv1-plugin-analysis	32.43% <ø> (ø)
.-pkg-app-pipedv1-plugin-ecs	31.82% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	58.37% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	61.90% <ø> (ø)
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	37.95% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	33.04% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	52.71% <ø> (ø)
.-pkg-plugin-sdk	50.34% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.51% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[t-kikuc]

thank you!