[r] Fixing the CRAN submission issues

[saigiridhar21]

• Fixing the CRAN submission issues that will be complained, while releasing the package on CRAN package manager.

Issue 1: Dependence on the R version should not specify the patch version.
Issue 2: Ignoring the unrequired files and directories from being added to the R package.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

@Ramanth

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[Ramanth]

LGTM