docs: Add release notes for 2.19.1 (#6840)

Co-authored-by: robot-ci-heartex <[email protected]>
Co-authored-by: caitlinwheeless <[email protected]>

Author: robot-ci-heartex

docs/source/guide/release_notes/onprem/2.19.1.md

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+## Label Studio Enterprise 2.19.1
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+<div class="onprem-highlight">Bug fixes </div>
+
+*Jan 02, 2025*
+
+Helm Chart version: 1.7.4
+
+### Bug fixes
+- Fixed an issue where, in some cases, project roles were reset on SAML SSO login. 
+- Fixed an issue affecting Redis credentials with special characters. 
+