fix: disable excerpt more filter when excerpt block is used

[GrigoreMihai]

Summary

During the render_block_data filter we check if the rendered block is a core/post-excerpt block and we disable our excerpt addon for the first call after the render_block_data hook.

Will affect visual aspect of the product

NO

Screenshots

Duplicate read more is no longer present when the excert more block is used: https://vertis.d.pr/i/aqx1p7 and works as excepted in the rest of the cases: https://vertis.d.pr/i/uAYkUZ .

Test instructions

Check the instructions from the linked issue for replication. The read more should not be doubled anymore.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #4187.

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[irinelenache]

@GrigoreMihai Tested and the issue is fixed now 🚀

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🎉 This PR is included in version 3.8.3 🎉

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