Add docs about how to upgrade spring boot version (Azure#19785)

Author: Rujun Chen

eng/versioning/external_dependencies.txt

@@ -135,7 +135,6 @@ org.springframework:spring-web;5.3.4
 org.springframework:spring-webmvc;5.3.4
 org.springframework:spring-webflux;5.3.4
 
-# spring-boot-starter-parent is not managed by spring-boot-dependencies or spring-cloud-dependencies.
 org.springframework.boot:spring-boot-starter-parent;2.4.3
 
 ## Spring cloud dependency versions

sdk/spring/scripts/spring_managed_external_dependencies.txt

@@ -1060,6 +1060,7 @@ org.springframework.boot:spring-boot-starter-mail;2.4.3
 org.springframework.boot:spring-boot-starter-mustache;2.4.3
 org.springframework.boot:spring-boot-starter-oauth2-client;2.4.3
 org.springframework.boot:spring-boot-starter-oauth2-resource-server;2.4.3
+org.springframework.boot:spring-boot-starter-parent;2.4.3
 org.springframework.boot:spring-boot-starter-quartz;2.4.3
 org.springframework.boot:spring-boot-starter-reactor-netty;2.4.3
 org.springframework.boot:spring-boot-starter-rsocket;2.4.3

sdk/spring/scripts/update_spring_managed_external_dependencies.py

@@ -1,6 +1,12 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
-
+#
+# How to use this script to update spring-boot's version?
+# Here are the steps:
+# 1. Update ROOT_POMS' version manually.
+# 2. Run command "python .\sdk\spring\scripts\update_spring_managed_external_dependencies.py".
+# 3. Run command "python .\eng\versioning\update_versions.py --ut external_dependency --sr".
+# 4. Verify result by command ".\eng\versioning\pom_file_version_scanner.ps1".
 
 import argparse
 import os
@@ -18,6 +24,7 @@
 
 EXTERNAL_DEPENDENCIES_FILE = 'eng/versioning/external_dependencies.txt'
 ROOT_POMS = [
+    'org.springframework.boot:spring-boot-starter-parent;2.4.3',
     'org.springframework.boot:spring-boot-dependencies;2.4.3',
     'org.springframework.cloud:spring-cloud-dependencies;2020.0.1'
 ]