Add Release Note for removal of 'disable_configuration_modification'

[TesterTom2021]

The change to remove 'disable_configuration_modification' was not included in the ATS 9.0 Release Notes.

Here is the change:
#5530

Here are the Release Notes.
https://docs.trafficserver.apache.org/en/9.0.x/release-notes/upgrading.en.html#removed-records-config-settings

Please update the release notes to reflect that the 'proxy.config.disable_configuration_modification' setting has been removed from the records.config.

[bryancall]

@zwoop Can you please take a look at this.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. Marking it stale to flag it for further consideration by the community.

[maskit]

We should fix this on 9.2.0 release at the latest.