update readme for dotnet8

Author: LZY7977

README.md

@@ -47,6 +47,7 @@ The following images are actively maintained by AWS CodeBuild, and are listed in
 + [amazonlinux-x86_64-lambda-standard:corretto17](al-lambda/x86_64/corretto17)
 + [amazonlinux-x86_64-lambda-standard:corretto21](al-lambda/x86_64/corretto21)
 + [amazonlinux-x86_64-lambda-standard:dotnet6](al-lambda/x86_64/dotnet6)
++ [amazonlinux-x86_64-lambda-standard:dotnet8](al-lambda/x86_64/dotnet8)
 + [amazonlinux-x86_64-lambda-standard:go1.21](al-lambda/x86_64/go1.21)
 + [amazonlinux-x86_64-lambda-standard:nodejs18](al-lambda/x86_64/nodejs18)
 + [amazonlinux-x86_64-lambda-standard:nodejs20](al-lambda/x86_64/nodejs20)
@@ -57,6 +58,7 @@ The following images are actively maintained by AWS CodeBuild, and are listed in
 + [amazonlinux-aarch64-lambda-standard:corretto17](al-lambda/aarch64/corretto17)
 + [amazonlinux-aarch64-lambda-standard:corretto21](al-lambda/aarch64/corretto21)
 + [amazonlinux-aarch64-lambda-standard:dotnet6](al-lambda/aarch64/dotnet6)
++ [amazonlinux-aarch64-lambda-standard:dotnet8](al-lambda/aarch64/dotnet8)
 + [amazonlinux-aarch64-lambda-standard:go1.21](al-lambda/aarch64/go1.21)
 + [amazonlinux-aarch64-lambda-standard:nodejs18](al-lambda/aarch64/nodejs18)
 + [amazonlinux-aarch64-lambda-standard:nodejs20](al-lambda/aarch64/nodejs20)