[EXT_texture_compression_rgtc] Align with Web IDL specification

[autokagami]

🤖 This is an automated pull request to align the spec with the latest Web IDL specification. 🤖

The followings are the Web IDL validation messages, which may help understanding this PR:

• Validation error at line 2 in EXT_texture_compression_rgtc,0, inside `interface EXT_texture_compression_rgtc -> extended-attribute NoInterfaceObject`:
  [NoInterfaceObject]
   ^
  

  Error: [NoInterfaceObject] extended attribute is a legacy feature that is now renamed to [LegacyNoInterfaceObject]. Refer to the relevant upstream PR for more information.

• Validation error at line 3 in EXT_texture_compression_rgtc,0, inside `interface EXT_texture_compression_rgtc`:
  interface EXT_texture_compression_rgtc {
            ^
  

  Error: Interfaces must have [Exposed] extended attribute. To fix, add, for example, [Exposed=Window]. Please also consider carefully if your interface should also be exposed in a Worker scope. Refer to the WebIDL spec section on Exposed for more information.

Currently this autofix might introduce awkward code formatting, and feel free to manually fix it whenever it happens.

Please file an issue at https://github.com/saschanaz/webidl-updater/issues/new if you think this is invalid or should be enhanced.

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[kenrussell]

Looks like this one might still need to be handled @jdashg .

[kenrussell]

Handled in #3279 .