Add full Go 1.21 support

[eskultety]

Maintainers will complete the following section

This PR enables full Go 1.21 support by the means of instructing Go to download the desired toolchain viaGOTOOLCHAIN=auto. This is limited to 1.21 only, so 1.22 will be explicitly refused (1.22 will be added by a standalone PR).

Notes:

• first 3 patches are some generic tiny fixes

• EnvironmentVariable was adjusted to act as a more generic resolvable template (e.g. to allow GOPROXY=file://${GOMODCACHE}/foo)

• Go 1.21 full support is enabled using GOTOOLCHAIN and GOPROXY

• [x] Commit messages are descriptive enough

• [x] Code coverage from testing does not decrease and new code is covered

• [x] Docs updated (if applicable)

• [ ] Docs links in the code are still valid (if docs were updated)

Note: if the contribution is external (not from an organization member), the CI pipeline will not run automatically. After verifying that the CI is safe to run:

• approve GitHub Actions workflows by clicking a button
• approve the Red Hat Trusted App Pipeline container build by commenting /ok-to-test (as is the standard for Pipelines as Code)

[eskultety]

since v2:

• changed detection of variable resolution cycles per the review comments
• forced setting GOTOOLCHAIN=local for cases where go base version > go.mod version > 1.21 (@taylormadore this relates to https://github.com/containerbuildsystem/cachi2/issues/501#issuecomment-2008970964, do you think it will be enough for now?)

[eskultety]

since v3:

• GOTOOLCHAIN=auto is only used for dependency fetching, but it's not enforced upon users by the means of generate-env
• toolchain keyword in go.mod file is considered as well when checking the maximum supported version violation (due to the use of GOTOOLCHAIN=auto)
• container image is preinstalled with a static 1.21.0 Go SDK along with 1.20
• some minor debug log adjustments
• rebase on main

[eskultety]

since v4:

• add documentation for Go 1.21 full support

[eskultety]

@brunoapimentel @taylormadore sorry for the delay, please have another look at the changes. I'll work on some integration tests in the meantime.

[eskultety]

Since v5:

• fix integration tests by generating fresh mock data

[eskultety]

Since v6:

• docs changes as requested in review
• dropped explicit GOTOOLCHAIN=local setting in the generated env file to really let users to set the variable however they like
• moved the GOTOOLCHAIN=auto enablement to the correct commit

[eskultety]

Since v7:

• drop the go backwards compatibility footnote as it dropping was incorrect
• convert since cachi2-v0.6.0 from bold italic to just italic (typo)