[communication] Update examples to use string for retry-after (#29739)

- Required by autorest schema for examples

Author: natekimball-msft

specification/communication/data-plane/Email/preview/2021-10-01-preview/examples/GetSendStatus.json

@@ -8,7 +8,7 @@
   "responses": {
     "200": {
       "headers": {
-        "Retry-After": 100
+        "Retry-After": "100"
       },
       "body": {
         "messageId": "F9168C5E-CEB2-4faa-B6BF-329BF39FA1E4",

specification/communication/data-plane/Email/preview/2021-10-01-preview/examples/SendEmail.json

@@ -81,7 +81,7 @@
       "headers": {
         "Repeatability-Result": "accepted",
         "Operation-Location": "https://contoso.westus.communications.azure.com//email/status/8540c0de-899f-5cce-acb5-3ec493af3800?api-version=2021-10-01-preview",
-        "Retry-After": 1,
+        "Retry-After": "1",
         "x-ms-request-id": "8540c0de-899f-5cce-acb5-3ec493af3800"
       }
     }

specification/communication/data-plane/Email/preview/2023-01-15-preview/examples/GetOperationStatusReturnsNonTerminalStatus.json

@@ -8,7 +8,7 @@
   "responses": {
     "200": {
       "headers": {
-        "retry-after": 100
+        "retry-after": "100"
       },
       "body": {
         "id": "F9168C5E-CEB2-4faa-B6BF-329BF39FA1E4",

specification/communication/data-plane/Email/preview/2023-01-15-preview/examples/SendEmail.json

@@ -73,7 +73,7 @@
     "202": {
       "headers": {
         "Operation-Location": "https://contoso.westus.communications.azure.com//emails/operations/8540c0de-899f-5cce-acb5-3ec493af3800?api-version=2023-01-15-preview",
-        "retry-after": 20
+        "retry-after": "20"
       },
       "body": {
         "id": "8540c0de-899f-5cce-acb5-3ec493af3800",

specification/communication/data-plane/Email/stable/2023-03-31/examples/GetOperationStatusReturnsNonTerminalStatus.json

@@ -8,7 +8,7 @@
   "responses": {
     "200": {
       "headers": {
-        "retry-after": 100
+        "retry-after": "100"
       },
       "body": {
         "id": "F9168C5E-CEB2-4faa-B6BF-329BF39FA1E4",

specification/communication/data-plane/Email/stable/2023-03-31/examples/SendEmail.json

@@ -73,7 +73,7 @@
     "202": {
       "headers": {
         "Operation-Location": "https://contoso.westus.communications.azure.com//emails/operations/8540c0de-899f-5cce-acb5-3ec493af3800?api-version=2023-03-31",
-        "retry-after": 20
+        "retry-after": "20"
       },
       "body": {
         "id": "8540c0de-899f-5cce-acb5-3ec493af3800",