Change configuration of provider

[sel-bukharov]

What's changed

In this PR we change way to configure provider:

• auth_url and auth_region are now required to configure provider
• SEL_REGION and SEL_PROJECT_ID env variables are renamed to INFRA_REGION and INFRA_PROJECT_ID respectively

In addition, resources which was deprecated long time ago was permanently removed from provider:

• selectel_vpc_role_v2
• selectel_vpc_token_v2
• selectel_vpc_user_v2
• selectel_vpc_vrrp_subnet_v2
• selectel_vpc_crossregion_subnet_v2

Last but not least: changes such as updating deps and bumping Go version with updating necessary documentation (changelog, readme).

How to review changes

To make things easier, I've broken the PR into separate commits. If you want to check all the changes, I strongly recommend checking the commits sequentially.

Pay attention to commit messages

I have separate the commit messages meaningfully: the name in brackets help to identify the scope of the commit changes.

For instance:

• if you want to check all commit where documentation was changed then check the commits with docs in brackets.
• if you want to check commits for database resources check the commits with dbaas in brackets.

For each reviewer I suggest the following flow of review:

• Check out commits with your area of interest: craas, dbaas, mks, secretmanager, vpc, iam, docs
• Optionally check out provider and deps commits - that is core changes, which change way to configure provider or relate to dependencies updates (f.e. Go upgrade)