fix: remove dropped formats from SupportedExts

[veeceey]

Summary

• Remove HCL, Java Properties, and INI from SupportedExts — these formats were removed from core encoding in v1.20, but SupportedExts still listed them as "universally supported"
• Update README to list only the built-in formats (JSON, TOML, YAML, Dotenv) and add a note pointing users to the upgrade guide for the removed formats
• Update test that referenced hcl as a config type to use toml instead

Problem

Since v1.20 removed HCL, Java Properties, and INI from core encoding (DefaultCodecRegistry only supports JSON, TOML, YAML, Dotenv), having them in SupportedExts caused:

1. Misleading file searches: searchInPath and findConfigFile iterated over SupportedExts looking for .hcl, .ini, .properties files that Viper cannot decode without a custom codec
2. Confusing errors: The slices.Contains(SupportedExts, ...) validation passed for these formats, but the subsequent decoderRegistry.Decoder(format) call failed with a generic ConfigParseError instead of an UnsupportedConfigError
3. Outdated documentation: The README still listed INI and "Java Propeties" [sic] as supported formats

Changes

File	Change
viper.go	Remove properties, props, prop, hcl, tfvars, ini from SupportedExts (both the var declaration and Reset())
viper.go	Add doc comment explaining how to re-enable removed formats
README.md	Update supported formats list; add note with link to upgrade guide
viper_test.go	Change mismatch test from hcl to toml (still tests the same file-extension-takes-precedence behavior)

Backward compatibility

SupportedExts is an exported var that users can modify. Users who register custom codecs for HCL/INI/Properties via WithCodecRegistry will need to also append the corresponding extensions to SupportedExts. This is documented in the updated comment on the variable.

Test plan

• All existing tests pass (go test ./...)
• go vet ./... passes
• Verify that config file search no longer looks for unsupported extensions
• Verify that attempting to use a removed format produces UnsupportedConfigError instead of ConfigParseError

Fixes #2092

[CLAassistant]

All committers have signed the CLA.

[veeceey]

CI is green across all platforms and Go versions except for one flaky TestWatchFile/file_content_changed failure on windows-latest with Go 1.24 (viper_bind_struct build tag). This is a pre-existing race condition in the file watcher test on Windows -- unrelated to the SupportedExts changes in this PR. The same test passes on the other Windows/Go combinations. Ready for review when you get a chance.