[8.18] chore(login form): Fix layout for Login assistance message (#233497) (#233577)

# Backport

This will backport the following commits from `main` to `8.18`:
- [chore(login form): Fix layout for Login assistance message
(#233497)](#233497)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT
[{"author":{"name":"Sid","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-08-31T11:49:29Z","message":"chore(login
form): Fix layout for Login assistance message (#233497)\n\n##
Summary\n\nCloses
https://github.com/elastic/kibana/issues/227824\n\nFixes margin for EUI
Horizontal rule component when Login Assistance\nMessage is
present\n\n\n### Test\nRun kibana with the following setting in
kibana.(dev.)yaml\n```\nxpack.security.loginAssistanceMessage: Test
Login assistance message\n```\nOn main vs this PR branch, you should see
the difference as shown below.\n\n\n| Main | After fix
|\n|--------|--------|\n| <img width=\"500\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/ab00606f-aaf2-446f-9118-4f8ce30bf6bb\"\n/>
| <img width=\"500\" alt=\"Screenshot 2025-08-29 at 15 39
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form): Fix layout for Login assistance
message","number":233497,"url":"https://github.com/elastic/kibana/pull/233497","mergeCommit":{"message":"chore(login
form): Fix layout for Login assistance message (#233497)\n\n##
Summary\n\nCloses
https://github.com/elastic/kibana/issues/227824\n\nFixes margin for EUI
Horizontal rule component when Login Assistance\nMessage is
present\n\n\n### Test\nRun kibana with the following setting in
kibana.(dev.)yaml\n```\nxpack.security.loginAssistanceMessage: Test
Login assistance message\n```\nOn main vs this PR branch, you should see
the difference as shown below.\n\n\n| Main | After fix
|\n|--------|--------|\n| <img width=\"500\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/ab00606f-aaf2-446f-9118-4f8ce30bf6bb\"\n/>
| <img width=\"500\" alt=\"Screenshot 2025-08-29 at 15 39
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|","sha":"d2f64bc8c8a063b3c0109ab09a5759afd6620cdd"}},"sourceBranch":"main","suggestedTargetBranches":[],"targetPullRequestStates":[{"branch":"main","label":"v9.2.0","branchLabelMappingKey":"^v9.2.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/233497","number":233497,"mergeCommit":{"message":"chore(login
form): Fix layout for Login assistance message (#233497)\n\n##
Summary\n\nCloses
https://github.com/elastic/kibana/issues/227824\n\nFixes margin for EUI
Horizontal rule component when Login Assistance\nMessage is
present\n\n\n### Test\nRun kibana with the following setting in
kibana.(dev.)yaml\n```\nxpack.security.loginAssistanceMessage: Test
Login assistance message\n```\nOn main vs this PR branch, you should see
the difference as shown below.\n\n\n| Main | After fix
|\n|--------|--------|\n| <img width=\"500\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/ab00606f-aaf2-446f-9118-4f8ce30bf6bb\"\n/>
| <img width=\"500\" alt=\"Screenshot 2025-08-29 at 15 39
42\"\nsrc=\"https://github.com/user-attachments/assets/0bdc8948-00a5-4b5f-b4fb-52f0ae04709c\"\n/>
|","sha":"d2f64bc8c8a063b3c0109ab09a5759afd6620cdd"}}]}] BACKPORT-->

Co-authored-by: Sid <[email protected]>

Author: kibanamachine

x-pack/platform/plugins/shared/security/public/authentication/login/components/login_form/login_form.scss

@@ -51,6 +51,6 @@
 
 .secLoginAssistanceMessage {
   // This tightens up the layout if message is present
-  margin-top: -($euiSizeXXL + $euiSizeS);
+  margin-top: -($euiSizeXL);
   padding: 0 $euiSize;
 }