Add deprecation warning for TemplatedConfigLoader and ConfigLoader

[ankatiyar]

Description

Resolve #2693

Development notes

Added the deprecation warning in session.py when the context is loaded.

Alternate Approach

• Could also do this in the __init__ functions of ConfigLoader and TemplatedConfigLoader but those warnings show up multiple times because the config loader is also initialised when loading logging then later when context is created.
• Used FutureWarning instead of DeprecationWarning as it is visible by default (See Do not unsuppress DeprecationWarnings with Kedro #2747 and Avoid setting warning filters globally #2744)

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes

[SajidAlamQB]

LGTM! 🌟 Just left some minor suggestions, thank you @ankatiyar.

[SajidAlamQB]

Suggested change

	f"{type(config_loader).__name__} will be deprecated in Kedro 0.19."
	f" Please use the OmegaConfigLoader instead. To consult"
	f" the documentation for OmegaConfigLoader, see here:"
	f" https://docs.kedro.org/en/stable/configuration/advanced_configuration.html#omegaconfigloader", # pylint: disable=line-too-long
	f"{type(config_loader).__name__} will be deprecated in Kedro 0.19."
	f" Please use the OmegaConfigLoader instead."
	f" To import OmegaConfigLoader use: 'from kedro.config import OmegaConfigLoader'."
	f" Consult the documentation for OmegaConfigLoader at:"
	f" https://docs.kedro.org/en/stable/configuration/"
	f"advanced_configuration.html#omegaconfigloader",

[SajidAlamQB]

Minor suggestions (feel free to use or ignore):

• Maybe just split the link into multiple lines.
• Add the new import line for OmegaConfigLoader for convenience.

[ankatiyar]

Thanks Sajid, I split the link into multiple lines but I think the import line will make it too verbose. The link to the OmegaConfigLoader docs has this right on top. :)

[merelcht]

LGTM 👍 Don't forget to add this to the release notes. This is an important deprecation that are users should be made aware of.