Add deprecation warnings to v3

[GuySartorelli]

This was originally part of #228

As a developer maintaining a project using LinkField v2/v3, I have a a clear path and clear guidance on how to migrate to v4.

Acceptance Criteria

• Deprecations warnings are added to v3 for any public API which was removed in v4
• The upgrade guide is updated to say to upgrade to (either 3.x-dev, 3.0.0, or 3.0.0-beta2 depending on the below) and resolve any deprecation warnings before migrating.

To be decided by @maxime-rainville:

• Do we want to tag a 3.0.0-beta2 with the deprecations? Or just leave them on the 3 branch? Or tag stable?

PR

• API Add deprecations #269
• DOC Enable deprecations for linkfield v3 #270

[maxime-rainville]

I think we need to tag something. My preference would be to tag 3.0.0 stable.

If that's not practical, 3.0.0-beta2 will work.

[GuySartorelli]

PRs merged, reassigned to Steve for whatever's left

[emteknetnz]

Assigning to @maxime-rainville to decide what to do regarding tagging

[maxime-rainville]

Unless someone is telling me that tagging 3.0.0 stable is not advisable, that is still my preferred option.

[emteknetnz]

3.0.0 stable was released https://github.com/silverstripe/silverstripe-linkfield/releases/tag/3.0.0