[Monolog Bridge] Remove deprecated log methods + add unit tests
feature #12731  [Monolog Bridge] Remove deprecated log methods + add unit tests (FlorianLB)

This PR was squashed before being merged into the 3.0-dev branch (closes #12731).

Discussion
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 [Monolog Bridge] Remove deprecated log methods + add unit tests

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | --
| License       | MIT
| Doc PR        | --

Remove deprecated methods. I took the opportunity to add some missing unit tests.

Commits
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9ecfc84  [Monolog Bridge] Remove deprecated log methods + add unit tests