…tions to section block

…`/`verbatim` options to section block (cvergne)

This PR was squashed before being merged into the 7.2 branch.

Discussion
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[Notifier] [Slack] Add button block element and `emoji`/`verbatim` options to section block

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Issues        | -
| License       | MIT

Hi,

When using the Slack Notifier bridge, I've noticed it wasn't possible to add a button as accessory to a section because button is only managed in the SlackActionsBlock, which is not allowed as an accessory in a Section block.
So the first purpose of this PR is add a SlackButtonBlockElement we can use as `accessory` into a section block (and use that block into SlackActionsBlock as it follows the same structure).

Then, I noticed `verbatim` (for markdown) and `emoji` (for plain text) options were not available in SectionBlock (but available in ContextBlock), so I've added them.
Note that originally, I was going to add them as optional arguments (`?bool $verbatim = null`) to avoid adding them if not explicitly given and keep the Slack default value, but the ContextBlock adds a default value (the same as Slack) for them, so I kept the same argument signature.

You can see an example of both use of button as accessory and verbatim param with the following link on [Slack Block Kit Builder](https://app.slack.com/block-kit-builder/#%7B%22blocks%22:%5B%7B%22type%22:%22section%22,%22text%22:%7B%22type%22:%22mrkdwn%22,%22text%22:%22Example%20with%20URL%20:%20Symfony.com%22,%22verbatim%22:true%7D,%22accessory%22:%7B%22type%22:%22button%22,%22text%22:%7B%22type%22:%22plain_text%22,%22emoji%22:true,%22text%22:%22Button%20as%20accessory%22%7D,%22value%22:%22https://symfony.com%22%7D%7D,%7B%22type%22:%22context%22,%22elements%22:%5B%7B%22type%22:%22mrkdwn%22,%22text%22:%22from%20*Symfony.com*%22,%22verbatim%22:true%7D%5D%7D%5D%7D) (_must be logged in to a Slack Account_).

_These minor additions are in the same PR, but tell me if I should split them into two PRs._

Thanks :)

Commits
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e9b79542a3 [Notifier] [Slack] Add button block element and `emoji`/`verbatim` options to section block

… emoji/verbatim options to section block

* 7.1:
  Remove subtree split checks
  Remove subtree split checks
  Revert "minor #54653 Auto-close PRs on subtree-splits (nicolas-grekas)"
  [SecurityBundle] Fix `container.build_hash` parameter binding
  [Serializer] Fix denormalizing a collection of union types
  [DoctrineBridge] Fix `UniqueEntityValidator` with proxy object

…k element and emoji/verbatim options (cvergne)

This PR was merged into the 7.2 branch.

Discussion
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[Notifier] [Slack] Add README examples about button block element and emoji/verbatim options

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | Fix symfony/symfony-docs#19929
| License       | MIT

This PR is just a followup of my previously merged PR symfony/symfony#54737.
Following the merge, [an issue was created about documentation](symfony/symfony-docs#19929), but each Notifier Bridge documentation is directly in README of each bridge so here it is :)

Commits
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1a52e8717f [Notifier] [Slack] Add README examples about button block element and emoji/verbatim options to section block

* 7.1:
  Add PR template and auto-close PR on subtree split repositories

…buh)

This PR was merged into the 7.2 branch.

Discussion
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[Notifier] deprecate the TransportFactoryTestCase

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | yes
| Issues        | Fix #54100
| License       | MIT

Commits
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c8b0fcc77b deprecate the TransportFactoryTestCase