Remove deprecated code in CMS5 for core

[maxime-rainville]

Acceptance criteria

• Code from the 1/4 branches is merged-up for core modules first
• All deprecated code is removed for all core modules
• All unit tests covering deprecated code are removed
• YML Config is reviewed to make sure it doesn't reference removed code
• A list of all deprecated methods and classes is added to the CMS5 changelog to help people plan their upgrades
• recipe-plugin and vendor-plugin are correctly updated (they lack Deprecation::notice())
• graphql v3 _legacy folders are deleted from asset-admin, elemental, versioned-admin and any others e.g. cms
• Remove the Serializable interface from classes - https://github.com/silverstripeltd/product-issues/issues/626

Passed tests in CI for silverstripe/installer

• https://github.com/sabina-talipova/silverstripe-installer/actions/runs/3635513990/jobs/6134681972

PR

• API Remove deprecated code silverstripe-admin#1393
• API Remove deprecated code silverstripe-asset-admin#1307
• API Remove deprecated code silverstripe-assets#530
• API Remove deprecated code silverstripe-behat-extension#234
• API Remove deprecated code silverstripe-campaign-admin#249
• API Remove deprecated code silverstripe-cms#2800
• API Remove deprecated code #10594
• API Remove deprecated code silverstripe-frameworktest#130
• API Remove deprecated code silverstripe-graphql#510
• API Remove deprecated code recipe-plugin#30
• API Remove deprecated code vendor-plugin#63
• API Remove deprecated code silverstripe-versioned#381
• API Remove deprecated code silverstripe-versioned-admin#265
• API Remove deprecated code silverstripe-config#75

More PRs

• https://github.com/emteknetnz/silverstripe-installer/actions/runs/3643257008 installer run
• API Restore nullable params silverstripe-config#76

[GuySartorelli]

Why would the list of deprecated stuff go to userhelp? Content authors shouldn't be expected to know or care about this.

[sabina-talipova]

Merged-up branches:

• silverstripe-testsession;
• silverstripe-behat-extension
• silverstripe-queuedjobs
• silverstripe-gridfieldextensions
• vendor-plugin
• silverstripe-registry
• silverstripe-elemental
• silverstripe-graphql
• silverstripe-config
• silverstripe-cms
• silverstripe-campaign-admin
• silverstripe-asset
• silverstripe-asset-admin
• silverstripe-admin
• silverstripe-versioned-admin
• silverstripe-versioned
• silverstripe-siteconfig
• developer-docs
• silverstripe-framework

[sabina-talipova]

Deprecated code was removed. Related to this issue tickets were created. Close this issue.