[DOC] README indicates the lowest supported TypeScript version

[tbouffard]

Put the information in the README. This is a first step of larger changes that will be implemented later.
Making it explicit in the README is just a minimum.

refs #2246

[assynour]

From what I understood in #2246, it is TS 4.5 for 0.26.1. This does not apply for older releases, right? should we explicitly mention something like "starting from 0.26.1..."?

[tbouffard]

From what I understood in #2246, it is TS 4.5 for 0.26.1. This does not apply for older releases, right? should we explicitly mention something like "starting from 0.26.1..."?

The README is always about the latest version.
That being said, I can add a note about the latest bpmn-visualization version that support previous TypeScript version. That can be helpful until we implement #2252
In #2246, I have notice that TS 4.5 has been required since v0.21.0. In #2221, a user mention they have to use v0.20.0 with Angular 11 which doesn't support TS 4.5.

I am going to do some tests in the example repository to confirm it

[tbouffard]

@assynour previous versions could run with older TypeScript version but the types have major issues that we fixed in https://github.com/process-analytics/bpmn-visualization-js/releases/tag/v0.21.5.
For instance, I can use TypeScript 4.1.6 with bpmn-visualization 0.19.5, but I got plenty of errors that are fixed by #1814.
And I don't know what TS version older versions can be run with.

For all these reasons, I don't want to add more information in the README and I suggest we keep only valid information for the current release.