Bootstrap 5 support

[internalsystemerror]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Update bootstrap to v5

What is the current behavior? (You can also link to an open issue here)
Bootstrap v4 is used

What is the new behavior (if this is a feature change)?
Bootstrap v5 is used

Please check if the PR fulfills these requirements

• The commit messages follow our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• A unit test has been written for this change.
• Running npm run build produced a successful build. (Unit testing can be done by running npm test;)
• My code has been linted. (npm run lint to do this. npm run build will fail if there are files not linted.)

Please provide a screenshot of this feature before and after your code changes, if applicable.
N/A (visual changes should be none if possible)

Other information:
I had a go at implementing floating labels, however this would require the form control to be a direct sibling to the label, which I'm not sure is possible currently. Alternatively we could include the entire floating label css but rewritten (the first option would be ideal however).

Closes #2832.

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[internalsystemerror]

I believe this is now ready for review. There is a failing test in core but I believe this to have already been failing before the changes involved in this PR, since this is based on the main branch.

[aitboudad]

👍, I think we need to mention the new change in our UPGRADE file

[internalsystemerror]

@aitboudad Added the upgrade documentation (I left a blank space for core docs if needed) and updated the README ready for ng12.

(This assumes v6 is targeting n11+12, and v7 is targeting ng12 + bootstrap5 which is probably incorrect so let me know and I'll push the fix)

[manuelmeister]

I think this should be done in formly v6 not v7, because v6 is currently just a pre-release.

[internalsystemerror]

Do you not want bootstrap 4 users to be able to use ng12 though? Can v5 already use Angular 12?

[aitboudad]

this feature will be added in V6 not V7.

Can v5 already use Angular 12?

it should work I think

[internalsystemerror]

I will update the PR accordingly

[internalsystemerror]

I believe this is now ready.

[Allcharles]

this feature will be added in V6 not V7.

Can v5 already use Angular 12?

it should work I think

Has this PR been merged into a pre-release deploy for V6 yet? Bootstrap 5 has been out for a while now and I would love to begin the process of migrating my website.

[aitboudad]

Has this PR been merged into a pre-release deploy for V6 yet?

it'll be included in our next alpha

[aitboudad]

Thank you @internalsystemerror!

[Wanchai]

Hi, thanks for your great work!

We need a new release of version6 to support Bootstrap 5.

Thanks in advance!

[aitboudad]

I'll make sure to make a new release this week which include this feature

[aitboudad]

Bootstrap 5 support is released in https://github.com/ngx-formly/ngx-formly/releases/tag/v6.0.0-next.4, we'll switch into beta release within 1/2 weeks.