apm-server: Remove server information endpoint POST / support (#647)

Corresponding code change:
elastic/apm-server#15976
Issue elastic/apm-server#15965

Author: carsonip

release-notes/breaking-changes/elastic-apm.md

@@ -17,12 +17,19 @@ To learn how to upgrade, check out <uprade docs>.
 % **Action**<br> Steps for mitigating deprecation impact.
 % ::::
 
-% ## 9.0.0 [elastic-apm-900-breaking-changes]
-% **Release date:** March 25, 2025
+## 9.0.0 [elastic-apm-9-0-0-breaking-changes]
+**Release date:** March 25, 2025
 
 % ::::{dropdown} Title of breaking change 
 % Description of the breaking change.
 % For more information, check [PR #](PR link).
 % **Impact**<br> Impact of the breaking change.
 % **Action**<br> Steps for mitigating deprecation impact.
-% ::::
+% ::::
+
+::::{dropdown} Change server information endpoint "/" to only accept GET and HEAD requests
+This will surface any agent misconfiguration causing data to be sent to `/` instead of the correct endpoint (for example, `/v1/traces` for OTLP/HTTP).
+For more information, check [PR #15976](https://github.com/elastic/apm-server/pull/15976).
+**Impact**<br> Any methods other than `GET` and `HEAD` to `/` will return HTTP 405 Method Not Allowed.
+**Action**<br> Update any existing usage, for example, update `POST /` to `GET /`.
+::::

solutions/observability/apps/apm-server-information-api.md

@@ -16,11 +16,11 @@ This is the server information endpoint:
 http(s)://{hostname}:{port}/
 ```
 
-Sending an `HTTP GET` or `HTTP POST` request to the server information endpoint will return an HTTP 200, indicating that the server is up.
+Sending an `HTTP GET` request to the server information endpoint will return an HTTP 200, indicating that the server is up.
 
 To configure authenticated access to the APM server, the instructions at [APM API key](api-keys.md) or [APM Secret Token](secret-token.md), must be followed to configure the correct permissions for APM access.
 
-If an [API keys](api-keys.md) or a [Secret token](secret-token.md) is passed along with the `HTTP GET` or `HTTP POST` request, in addition to an HTTP 200, the response payload will include some information about the APM server.
+If an [API keys](api-keys.md) or a [Secret token](secret-token.md) is passed along with the `HTTP GET` request, in addition to an HTTP 200, the response payload will include some information about the APM server.
 
 
 ### Example: GET, without credentials [apm-api-info-example-get-without-credentials] 
@@ -48,12 +48,12 @@ curl --verbose -X GET http://127.0.0.1:8200
 ```
 
 
-### Example: POST, with secret token [apm-api-info-example-post-with-secret-token] 
+### Example: GET, with secret token [apm-api-info-example-get-with-secret-token]
 
-Example APM Server information request with POST, with a [Secret token](secret-token.md):
+Example APM Server information request with GET, with a [Secret token](secret-token.md):
 
 ```sh
-curl -X POST http://127.0.0.1:8200/ \
+curl -X GET http://127.0.0.1:8200/ \
   -H "Authorization: Bearer secret_token"
 
 {