Deprecate levelpad of Logger.Formatter

[nallwhy]

This PR fixes#11629.

• $levelpad is removed from @default_pattern of Logger.Formatter
• Functions related to levelpad are annotated with @deprecated

[josevalim]

Thank you for the PR @nallwhy!

Unfortunately the @deprecated attributes you added won't work as expected. You need to remove them. Instead, you need to add something here:

elixir/lib/logger/lib/logger/formatter.ex

Line 102 in 2c3e902

defpcompile_code(key)whenkeyin@valid_patterns,do: key

Something like:

defpcompile_code(:levelpad)doIO.warn"$levelpad in Logger message format is deprecated, please remove it":levelpadend

[josevalim]

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