refactor: migrate getUrlWithBackToParam from no-restricted-syntax to @deprecated

Mark getUrlWithBackToParam with @deprecated JSDoc so usages are visible
via npm run eslint-report. Remove the no-restricted-syntax custom rule and
its 181 disable comments, and seatbelt the 176 existing call sites under
@typescript-eslint/no-deprecated so they are grandfathered without blocking CI.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: roryabraham

config/eslint/eslint.config.mjs

@@ -402,11 +402,6 @@ const config = defineConfig([
                     selector: 'CallExpression[callee.object.name="React"][callee.property.name="forwardRef"]',
                     message: 'forwardRef is deprecated. Please use ref as a prop instead. See: contributingGuides/STYLE.md#forwarding-refs',
                 },
-                {
-                    selector: 'CallExpression[callee.name="getUrlWithBackToParam"]',
-                    message:
-                        'Usage of getUrlWithBackToParam function is prohibited. This is legacy code and no new occurrences should be added. Please look into the `How to remove backTo from URL` section in contributingGuides/NAVIGATION.md. and use alternative routing methods instead.',
-                },
                 {
                     selector: 'ImportNamespaceSpecifier[parent.source.value=/^@libs/]',
                     message: 'Namespace imports from @libs are not allowed. Use named imports instead. Example: import { method } from "@libs/module"',

config/eslint/eslint.seatbelt.tsv

@@ -18,6 +18,7 @@
 "../../src/Expensify.tsx"	"react-hooks/set-state-in-effect"	1
 "../../src/GlobalModals.tsx"	"no-restricted-syntax"	2
 "../../src/ONYXKEYS.ts"	"no-restricted-syntax"	2
+"../../src/ROUTES.ts"	"@typescript-eslint/no-deprecated/getUrlWithBackToParam"	176
 "../../src/components/AccountingConnectionConfirmationModal.tsx"	"@typescript-eslint/no-deprecated/ConfirmModal"	1
 "../../src/components/ActionSheetAwareScrollView/useActionSheetAwareScrollViewRef.ts"	"react-hooks/immutability"	1
 "../../src/components/ActionSheetAwareScrollView/useActionSheetKeyboardSpacing.ts"	"@typescript-eslint/no-deprecated/useScrollViewOffset"	1