chore: bump starter version to 20260331 across Docker Compose examples

[hmoreras]

Summary

• Updates CUSTOM_STARTER_URL across all Docker Compose example configurations from version 20260326 to 20260331
• Keeps example configurations aligned with the latest starter release
• Follows the same pattern as chore: bump starter version to 20260326 across Docker Compose examples #35135

Closes #34732

This PR fixes: #34732

[claude]

Claude finished @hmoreras's task in 37s —— View job

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Rollback Safety Analysis

• Read rollback-unsafe categories reference
• Get full PR diff
• Analyze changes against unsafe categories
• Apply appropriate label → AI: Safe To Rollback

Result: ✅ Safe to Rollback

The changes in this PR are exclusively Docker Compose YAML configuration updates — bumping CUSTOM_STARTER_URL from 20260326 to 20260331 across 10 example files.

Checked against all unsafe categories:

Category	Match?
C-1 Structural Data Model Change	✅ No
C-2 Elasticsearch Mapping Change	✅ No
C-3 Content JSON Model Version Bump	✅ No
C-4 DROP TABLE/Column	✅ No
H-1 One-Way Data Migration	✅ No
H-2 RENAME TABLE/COLUMN	✅ No
H-3 PK Restructuring	✅ No
H-4 New ContentType Field Type	✅ No
H-5 Binary Storage Provider Change	✅ No
H-6 DROP PROCEDURE/FUNCTION	✅ No
H-7 NOT NULL Column Without Default	✅ No
M-1 Column Type Change	✅ No
M-2 Push Publishing Bundle Format	✅ No
M-3 REST/GraphQL API Contract Change	✅ No
M-4 OSGi API Breakage	✅ No

No database migrations, no Java code changes, no schema changes, no API contract changes. 9 of 10 changes are even commented-out lines. Rolling back to the previous release has no impact from these changes.