[Reader Customization] Add [internal] tag to the release note item

[thomashorta]

Add an [internal] tag to the Release Notes file entry for Reader Customization, since we won't need it to be shown in the Play Store notes for now, as discussed here: p1712614130772109/1712583344.843729-slack-CC7L49W13.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 24.6 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

	1 Message
📖	This PR contains changes to RELEASE-NOTES.txt. Note that these changes won't affect the final version of the release notes as this version is in code freeze. Please, get in touch with a release manager if you want to update the final release notes.

Generated by 🚫 Danger

[RenanLukas]

LGTM

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[wpmobilebot]

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	App Name	WordPress
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	Build Type	Debug
	Version	pr20613-320b42c
	Commit	320b42c
	Direct Download	wordpress-prototype-build-pr20613-320b42c.apk

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr20613-320b42c
	Commit	320b42c
	Direct Download	jetpack-prototype-build-pr20613-320b42c.apk

Note: Google Login is not supported on these builds.