Remove HEADER_KEYWORD & IMPL_KEYWORD

[3flex]

Description

Remove calls to KtTokens.HEADER_KEYWORD & KtTokens.IMPL_KEYWORD. These have been removed from Kotlin effective Kotlin 2.1.0-Beta1.

Without this change, projects that use ktlint-rule-engine-core as a dependency cannot build when upgraded to Kotlin-2.1.0-Beta1.

References:

• https://youtrack.jetbrains.com/issue/KT-52315/Legacy-keywords-header-impl-break-enum-definitions
• JetBrains/kotlin@76592dc#diff-acd77bf153b196d3b53b21fcd27f3971151fba10645714064933d6a916b2a4b2

Checklist

Before submitting the PR, please check following (checks which are not relevant may be ignored):

• Commit message are well written. In addition to a short title, the commit message also explain why a change is made.
• At least one commit message contains a reference Closes #<xxx> or Fixes #<xxx> (replace<xxx> with issue number)
• Tests are added
• KtLint format has been applied on source code itself and violations are fixed
• PR title is short and clear (it is used as description in the release changelog)
• PR description added (background information)

Documentation is updated. See difference between snapshot and release documentation

• Snapshot documentation in case documentation is to be released together with a code change
• Release documentation in case documentation is related to a released version of ktlint and has to be published as soon as the change is merged to master