Stabilize module configmiddleware

[mx-psi]

Before stabilizing a module, an approver or maintainer must make sure that the following criteria have been met for at least two successive minor version releases (regardless of when this issue was opened):

• No open issues or PRs in the module that would require breaking changes
• No TODOs in the module code that would require breaking changes
• No deprecated symbols in the module
• No symbols marked as experimental in the module
• The module follows the Coding guidelines

Please also make sure to publicly announce our intent to stabilize the module on:

• The #otel-collector CNCF Slack Channel
  • https://cloud-native.slack.com/archives/C01N6P7KR6W/p1756222820464179
• The #opentelemetry CNCF Slack channel
  • https://cloud-native.slack.com/archives/CJFCJHG4Q/p1756222842980649
• A Collector SIG meeting (if unable to attend, just add to the agenda)
  • Added to the agenda for August 26th

To help other people verify the above criteria, please link to the announcement and other links used to complete the above in a comment on this issue.

Once all criteria are met, close this issue by moving this module to the stable module set.

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@jmacd I am assigning this to you since you are actively working on this module.

[mx-psi]

No relevant uses: https://github.com/search?q=go.opentelemetry.io%2Fcollector%2Fconfig%2Fconfigmiddleware+language%3AGo+-is%3Afork+-org%3Aopen-telemetry&type=code

However this is

1. quite simple (it's only the configuration part, not the internal logic for middlewares)
2. we intend to use it
3. it's needed for stabilizing confighttp/configgrpc