feat: Make multiple roles per group/user GA by removing the flag (#5109)

To prepare for 5.6 GA, 
I've done a find through both Frontend and Backend here to remove the
usages of the flag. Seems like the flag was only in use in the frontend.
@nunogois can you confirm?

frontend/src/component/project/ProjectAccess/ProjectAccessAssign/ProjectAccessAssign.tsx

@@ -441,29 +441,12 @@ export const ProjectAccessAssign = ({
                             Select the role to assign for this project
                         </StyledInputDescription>
                         <StyledAutocompleteWrapper>
-                            <ConditionallyRender
-                                condition={Boolean(
-                                    uiConfig.flags.multipleRoles,
-                                )}
-                                show={() => (
                             <MultipleRoleSelect
                                 data-testid={PA_ROLE_ID}
                                 roles={roles}
                                 value={selectedRoles}
                                 setValue={setRoles}
                             />
-                                )}
-                                elseShow={() => (
-                                    <RoleSelect
-                                        data-testid={PA_ROLE_ID}
-                                        roles={roles}
-                                        value={selectedRoles[0]}
-                                        setValue={(role) =>
-                                            setRoles(role ? [role] : [])
-                                        }
-                                    />
-                                )}
-                            />
                         </StyledAutocompleteWrapper>
                     </div>
 

src/lib/__snapshots__/create-config.test.ts.snap

@@ -106,7 +106,6 @@ exports[`should create default config 1`] = `
         },
       },
       "migrationLock": true,
-      "multipleRoles": false,
       "personalAccessTokensKillSwitch": false,
       "playgroundImprovements": false,
       "privateProjects": false,
@@ -151,7 +150,6 @@ exports[`should create default config 1`] = `
         },
       },
       "migrationLock": true,
-      "multipleRoles": false,
       "personalAccessTokensKillSwitch": false,
       "playgroundImprovements": false,
       "privateProjects": false,

src/lib/types/experimental.ts

@@ -23,7 +23,6 @@ export type IFlagKey =
     | 'filterInvalidClientMetrics'
     | 'lastSeenByEnvironment'
     | 'customRootRolesKillSwitch'
-    | 'multipleRoles'
     | 'featureNamingPattern'
     | 'doraMetrics'
     | 'variantTypeNumber'
@@ -117,10 +116,6 @@ const flags: IFlags = {
         process.env.UNLEASH_EXPERIMENTAL_CUSTOM_ROOT_ROLES_KILL_SWITCH,
         false,
     ),
-    multipleRoles: parseEnvVarBoolean(
-        process.env.UNLEASH_EXPERIMENTAL_MULTIPLE_ROLES,
-        false,
-    ),
     featureNamingPattern: parseEnvVarBoolean(
         process.env.UNLEASH_EXPERIMENTAL_FEATURE_NAMING_PATTERN,
         false,