docs: update commit message guidelines to reflect current requirements (#43650)
docs: update commit message guidelines to reflect current requirements (#43650)

Update the commit message guidelines in `CONTRIBUTING.md` to reflect
recent changes in our dev-infra tooling:
- Remove the mention of the 100 chars/line limit, since that is
  [no longer in effect][1].
- Mention Deprecation sections:
  According to our [Deprecation practices][2], we should document
  deprecations in the Changelog, which our tooling supports via
  [DEPRECATED][3] sections in commit messages. Yet this was not
  mentioned in the commit message guidelines.

[1]: https://github.com/angular/angular/blob/a07cb097e9124738880f7978658a2972d25b892e/.ng-dev/commit-message.ts#L7
[2]: https://angular.io/guide/releases#deprecation-practices
[3]: https://github.com/angular/dev-infra/blob/b51360a7cc39cb3deb5ae1bf0bfb121fe7ee0a4e/ng-dev/commit-message/parse.ts#L77

PR Close #43650

angular-indonesia/angular,shairez/angular,ocombe/angular,gnomeontherun/angular,shairez/angular,ocombe/angular,ocombe/angular,angular/angular,brandonroberts/angular,ocombe/angular,angular-indonesia/angular,brandonroberts/angular,JiaLiPassion/angular,gkalpak/angular,manekinekko/angular,brandonroberts/angular,shairez/angular,JiaLiPassion/angular,gkalpak/angular,shairez/angular,gkalpak/angular,brandonroberts/angular,manekinekko/angular,angular-indonesia/angular,gnomeontherun/angular,manekinekko/angular,brandonroberts/angular,JiaLiPassion/angular,angular-indonesia/angular,angular/angular,angular/angular,ocombe/angular,manekinekko/angular,gkalpak/angular,angular/angular,brandonroberts/angular,JiaLiPassion/angular,manekinekko/angular,shairez/angular,gnomeontherun/angular,manekinekko/angular,ocombe/angular,gnomeontherun/angular