chore: bump step-security/harden-runner to v2.19.0 (#3266)

## Summary
- Bump `step-security/harden-runner` to `v2.19.0` (SHA
`8d3c67de8e2fe68ef647c8db1e6a09f647780f40`), the latest release
(2026-04-20).
- Replaces older pins so all workflows in this repo meet the ≥v2.17.0
org hardening bar.

Automated bump as part of an org-wide audit; no config flags changed.

## Test plan
- [ ] CI passes on this PR
- [ ] harden-runner initializes with the existing `egress-policy` in
each workflow

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antitree

.github/workflows/templates/export-courses-docs-to-gcs.yaml

@@ -24,7 +24,7 @@ jobs:
 
     steps:
     - name: Harden Runner
-      uses: step-security/harden-runner@fa2e9d605c4eeb9fcad4c99c224cee0c6c7f3594 # v2.16.0
+      uses: step-security/harden-runner@8d3c67de8e2fe68ef647c8db1e6a09f647780f40 # v2.19.0
       with:
         egress-policy: audit
 

.github/workflows/templates/export-docs-to-gcs.yaml

@@ -26,7 +26,7 @@ jobs:
 
     steps:
     - name: Harden Runner
-      uses: step-security/harden-runner@fa2e9d605c4eeb9fcad4c99c224cee0c6c7f3594 # v2.16.0
+      uses: step-security/harden-runner@8d3c67de8e2fe68ef647c8db1e6a09f647780f40 # v2.19.0
       with:
         egress-policy: audit
 

.github/workflows/templates/export-images-docs-to-gcs.yaml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
     - name: Harden Runner
-      uses: step-security/harden-runner@fa2e9d605c4eeb9fcad4c99c224cee0c6c7f3594 # v2.16.0
+      uses: step-security/harden-runner@8d3c67de8e2fe68ef647c8db1e6a09f647780f40 # v2.19.0
       with:
         egress-policy: audit