Change the 2.1.x branch to support Drupal < 10.3 for a smoother update process

Problem/Motivation

Faced issues with Drupal 9 and <= 10.2 after
#3429050: Automated Drupal 11 compatibility fixes for bootstrap_layout_builder
#3474611: Change to require at least Drupal 10.3

Proposed resolution

• Start the 2.2.x branch from the 2.x branch, and release 2.2.0
• Start the 2.1.x branch from the 2.x branch
• Change to core_version_requirement: <10.3 in the bootstrap_layout_builder.info.yml file. In the 2.1.x
• Change drupal/core to <10.3 in the composer.json file.In the 2.1.x
• Revert the following changes on the 2.1.x branch, but keep them in the 2.2.x branch

RenderElement and FormElement base plugin classes are deprecated and renamed to RenderElementBase and FormElementBase
#3431452: Rename RenderElement and FormElement plugin abstract classes to RenderElementBase and FormElementBase

Better to update to Drupal ^10.3
But doing this with the smoother update process.

Remaining tasks

User interface changes

API changes

Data model changes