Update CHANGELOG.md for 10.3.4

[ci skip]

changelogs/unreleased/api-no-service-pw-output.yml

----
-title: Filter out sensitive fields from the project services API
-merge_request:
-author: Robert Schilling
-type: security

changelogs/unreleased/fix-import-rce.yml

----
-title: Fix RCE via project import mechanism
-merge_request:
-author:
-type: security

changelogs/unreleased/jej-fix-disabled-oauth-access.yml

----
-title: Prevent OAuth login POST requests when a provider has been disabled
-merge_request:
-author:
-type: security

changelogs/unreleased/milestones-finder-order-fix.yml

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-title: Prevent a SQL injection in the MilestonesFinder
-merge_request:
-author:
-type: security

changelogs/unreleased/projectfix.yml

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-title: Check user authorization for source and target projects when creating a merge
-  request.
-merge_request:
-author:
-type: security

changelogs/unreleased/security-10-3.yml

----
-title: Fix path traversal in gitlab-ci.yml cache:key
-merge_request:
-author:
-type: security

changelogs/unreleased/sh-migrate-can-push-to-deploy-keys-projects.yml

----
-title: Fix writable shared deploy keys
-merge_request:
-author:
-type: security