Don't disable FPC for maintenance, instead send "no cache" headers

[theCapypara]

Description (*)

This Pull Request removes the need to have a writable env.php while switching the maintenance mode. Previously Magento tried to disable the FPC when switching into maintenance mode. This removes that and instead, to maintain functionality, sets the "no-cache" headers for all frontend requests in maintenance mode (Cache-Control etc.). The effect is, that pages are not cached while in maintenance mode.

• Removes the observer for disabling and re-enabling the FPC during
  maintenande mode switch

• Adds a new observer, that adds the "no cache" headers for all HTTP
  requests during maintenance mode

Fixed Issues (if relevant)

1. Unable to enable maintenance mode when env.php is read only #24229: Unable to enable maintenance mode when env.php is read only

Manual testing scenarios (*)

With Builtin FPC:

1. Keep maintenance mode disabled, access a cacheable Magento page
   • Page should be cached and delivered by FPC on next request
2. Make the app/etc/env.php file read-only
3. Enable maintenance mode
   • This should now not fail, previously it complained about the env.php not being writable
4. Add the local IP to the maintenance whitelist
5. Flush the FPC
6. Access a cacheable Magento page
   • Page should not be cached
7. Disable maintenance again and repeat 1.

Repeat tests for Varnish as FPC.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)

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[sivaschenko]

@magento run Unit Tests

[theCapypara]

After a recommendation by @sivaschenko, I changed it so, that public headers aren't even set in maintenance mode.

[ihor-sviziev]

Please revert changes in this file as this is not observer and removing it will be backward incompatible. If it’s not used anymore - please mark it with tag @deprecated with description why it was deprecated.

[ihor-sviziev]

Please make it private

[magento-engcom-team]

Hi @ihor-sviziev, thank you for the review.
ENGCOM-6367 has been created to process this Pull Request

[engcom-Bravo]

✔️ QA Passed

[m2-assistant]

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